Sphinx-based documentation added

docs/README.md

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+# `mp-units` - Documentation
+
+## How to build?
+
+1. Install the requirements (Sphinx) with:
+
+    ```shell
+    pip3 install -r docs/requirements.txt
+    ```
+
+2. Install all dependencies with Conan for a developer's build:
+
+    ```shell
+    conan install .. -pr <your_conan_profile> -s compiler.cppstd=20 -e CONAN_RUN_TESTS=True -b outdated
+    ```
+
+3. Install Python 3
+4. Build the documentation with a regular CMake build
+
+
+## How to contribute?
+
+To make any contribution to **mp-units** documentation please fork this repository and open
+a Pull Request.
+
+### Style Guidelines
+
+This guidelines are just general good practices for the formatting and structure of the whole
+documentation and do not pretend to be a stopper for any helpful contribution. Any contribution
+that may include relevant information for **mp-units** users will always be welcomed.
+
+**mp-units** documentation is written in [reStructuredText](http://docutils.sourceforge.net/rst.html)
+and follows [reStructuredText Markup Specification](http://docutils.sourceforge.net/docs/ref/rst/restructuredtext.html).
+
+[Quick reStructuredText](http://docutils.sourceforge.net/docs/user/rst/quickref.html) is also
+used for reference.
+
+Any detail not covered by this guidelines will follow the aforementioned rules.
+
+#### Section titles
+
+Use section titles in this level of importance:
+
+```rst
+Section Title
+=============
+
+Subsection Title
+----------------
+
+Subsubsection Title
+^^^^^^^^^^^^^^^^^^^
+```
+
+#### Text emphasis/highlighting
+
+- **Bold text** to highlight important text:
+
+  ```rst
+  **mp-units** is a compile-time enabled Modern C++ library that provides compile-time dimensional
+  analysis and unit/quantity manipulation.
+  ```
+
+- *Italics* to refer to file names, directory names, and paths.
+
+  ```rst
+  Create Conan configuration file (either *conanfile.txt* or *conanfile.py*) in your project's
+  top-level directory...
+  ```
+
+- ``Inline literals`` to refer to the in examples that is not a part of the **mp-units** library:
+
+  ```rst
+  Let's assume that the user wants to implement an ``avg_speed`` function that will
+  be calculating the average speed based on provided distance and duration quantities.
+  ```
+
+#### Literal blocks
+
+Most of the C++ code examples should be provided as literal blocks after double `::` symbol:
+
+```rst
+For this dimension-specific concepts come handy again and with usage of C++20 generic
+functions our function can look as simple as::
+
+    constexpr Velocity auto avg_speed(Length auto d, Time auto t)
+    {
+      return d / t;
+    }
+```
+
+#### code-blocks
+
+Use code-blocks for exceptional cases like code samples in other languages or a need
+to emphasize specific lines of code:
+
+```rst
+Quantities of the same dimension can be easily added or subtracted with
+each other and the result will always be a quantity of the same dimension:
+
+.. code-block::
+    :emphasize-lines: 3-4
+
+    Length auto dist1 = 2q_m;
+    Length auto dist2 = 1q_m;
+    Length auto res1 = dist1 + dist2;
+    Length auto res2 = dist1 - dist2;
+```
+
+#### Indentation and line length
+
+Make sure all indentation is done with spaces. Normally 2 space indentation for bulleted lists
+and 4 space indentation for code blocks and RST directives contents:
+
+Do not leave any unnecessary or trailing spaces.