README updated

README.md

@@ -5,9 +5,32 @@
 
 # `mp-units` - A Units Library for C++
 
+## TL;DR
+
+```cpp
+constexpr Velocity auto avg_speed(Length auto d, Time auto t)
+{
+  return d / t;
+}
+
+int main()
+{
+  using namespace si::literals;
+  Velocity auto v1 = avg_speed(220km, 2h);
+  Velocity auto v2 = avg_speed(140mi, 2h);
+
+  std::cout << v1 << '\n';                                          // 110 km/h
+  std::cout << quantity_cast<si::metre_per_second>(speed) << '\n';  // 30.5556 m/s
+  std::cout << v2 << '\n';                                          // 70 mi/h
+}
+```
+
+Try it on [Compiler Explorer](https://godbolt.org/z/OVpkT1).
+
+
 ## Summary
 
-`Units` is a compile-time enabled Modern C++ library that provides compile-time dimensional
+`mp-units` is a compile-time enabled Modern C++ library that provides compile-time dimensional
 analysis and unit/quantity manipulation. The basic idea and design heavily bases on
 `std::chrono::duration` and extends it to work properly with many dimensions.
 
@@ -31,74 +54,120 @@ static_assert(1000 / 1s == 1kHz);
 static_assert(10km / 5km == 2);
 ```
 
-## Usage Overview
 
-The library framework consists of a few concepts: quantities, units, dimensions and their exponents. From the user's
-point of view the most important is a `quantity`.
+## Getting Started
 
-Quantity is a concrete amount of a unit for a specified dimension with a specific representation:
+The library framework consists of a few concepts: quantities, units, dimensions and their
+exponents. From the user's point of view the most important is a `quantity`.
+
+A quantity is a concrete amount of a unit for a specified dimension with a specific representation:
 
 ```cpp
-units::quantity<units::kilometre, double> d1(123);
-auto d2 = 123km;    // units::quantity<units::kilometre, std::int64_t>
+units::quantity<units::si::dim_length, units::si::kilometre, double> d(123);
 ```
 
-There are C++ concepts provided for each such quantity type:
+To simplify quantity creation the library provides helper aliases for quantities of different
+dimensions. Thanks to then the above example can be rewritten as follows:
 
 ```cpp
-template<typename T>
-concept Length = QuantityOf<T, length>;
+units::si::length<units::si::kilometre, double> d(123);
 ```
 
-With that we can easily write a function template like this:
+To simplify creations of compile-time known constants the library provides UDLs for each unit.
+Thanks to them the same code can be as simple as:
 
 ```cpp
-constexpr units::Velocity auto avg_speed(units::Length auto d, units::Time auto t)
+using namespace units::si::literals;
+auto d = 123km;    // units::length<units::si::kilometre, std::int64_t>
+```
+
+For brevity, the next examples will assume:
+
+```cpp
+using namespace units;
+```
+
+Let's assume that the user wants to write the following code:
+
+```cpp
+int main()
+{
+  using namespace si::literals;
+  auto v1 = avg_speed(220km, 2h);
+  auto v2 = avg_speed(140mi, 2h);
+  // ...
+}
+```
+
+`avg_speed` is a simple function calculating an average speed from a distance and duration. It can
+be implemented as:
+
+```cpp
+constexpr si::velocity<si::metre_per_second, int> avg_speed(si::length<si::metre> d, si::time<si::second> t)
 {
   return d / t;
 }
 ```
 
-Concepts usage in the function above guarantee correctness enforced in compile-time and the
-guarantee that no unneeded intermediate conversions will ever be applied no matter which
-units the user will decide to pass to such function template:
+However, this function will perform unnecessary intermediate conversions (from kilometers to meters,
+from hours to seconds, and from meters per second to kilometers per hour). To eliminate all that
+overhead we have to write a template function:
 
 ```cpp
-using namespace units;
-
-constexpr quantity<kilometre> distance(220);
-constexpr quantity<hour> time(2);
-constexpr Velocity speed = avg_speed(distance, time);
-
-static_assert(std::same_as<std::remove_cvref_t<decltype(speed)>, quantity<kilometre_per_hour>>);
-static_assert(speed.count() == 110);
+template<typename U1, typename R1, typename U2, typename R2>
+constexpr auto avg_speed(si::length<U1, R1> d, si::time<U2, R2> t)
+{
+  return d / t;
+}
 ```
 
-The units library also tries really hard to printing any quantity in the most user friendly fashion:
+This function will work for every SI unit and representation without any unnecessary overhead.
+It is also simple enough to ensure that the returned type is actually a velocity. However,
+it might not always be the case. For more complicated calculations we would like to ensure
+that we are returning a correct type and also inform the user of that fact in the function
+template interface. Also we might want to implement a truly generic function that will work
+efficiently not only with SI units but also with other systems of units like CGS. The solution
+to this are C++20 concepts and generic functions.
 
 ```cpp
-std::cout << speed << '\n';
-std::cout << quantity_cast<metre_per_second>(speed) << '\n';
-std::cout << avg_speed(140.mi, 2h) << '\n';
+constexpr Velocity auto avg_speed(Length auto d, Time auto t)
+{
+  return d / t;
+}
 ```
 
-Try it on [Compiler Explorer](https://godbolt.org/z/OVpkT1).
+The units library also tries really hard to print any quantity in the most user friendly
+fashion:
+
+```cpp
+int main()
+{
+  using namespace si::literals;
+  Velocity auto v1 = avg_speed(220km, 2h);
+  Velocity auto v2 = avg_speed(140mi, 2h);
+
+  std::cout << v1 << '\n';                                          // 110 km/h
+  std::cout << quantity_cast<si::metre_per_second>(speed) << '\n';  // 30.5556 m/s
+  std::cout << v2 << '\n';                                          // 70 mi/h
+}
+```
 
 
 ## Library design
 
-`mp-units` library design rationale and documentation can be found in [doc/DESIGN.md](doc/DESIGN.md)
+A detailed `mp-units` library design rationale and documentation can be found in
+[doc/DESIGN.md](doc/DESIGN.md)
 
 
 ## Repository structure
 
-That repository contains the following independent `cmake`-based projects:
-- `./src` - header-only project for `units`
-- `.` - project used for development needs that wraps `./src` project together with
-  usage examples and unit tests
-- `./test_package` - library installation and Conan package verification
+This repository contains three independent `cmake`-based projects:
+1. `./src` - header-only project containing whole `mp-units` library
+2. `.` - project used as an entry point for library development (it wraps `./src` project
+  together with usage examples and tests)
+3. `./test_package` - library installation and Conan package verification
 
-Please note that the projects depend on `cmake` git submodule in the `./cmake/common`
+NOTE: Please note that this repository depends on a git submodule in the `./cmake/common`
 subdirectory.
 
 
@@ -112,6 +181,16 @@ NOTE: This library as of now compiles correctly only with gcc-9.1 and newer.
 
 ## Release notes
 
+- 0.5.0 ???
+  - Major refactoring and rewrite of the library
+  - Units are now independent from dimensions
+  - Dimensions are now depended on units (base or coherent units are provided in a class template)
+  - Quantity gets a Dimension template parameter again (as unit does not provide information about
+    its dimension anymore)
+  - Added official CGS system support
+  - Added official data information system support
+  - Repository file tree cleanup
+
 - 0.4.0 Nov 17, 2019
   - Support for derived dimensions in `exp` added
   - Added `pow()` and `sqrt()` operations on quantities