docs: quantity_io.h described in docs

README.md

@@ -64,6 +64,7 @@ the below example for a quick preview of basic library features:
 #include <units/physical/si/derived/speed.h>
 #include <units/physical/si/international/derived/speed.h>
 #include <units/format.h>
+#include <units/quantity_io.h>
 #include <iostream>
 
 using namespace units::physical;

docs/CHANGELOG.md

@@ -2,6 +2,7 @@
 
 - **0.7.0 WIP**
   - (!) refactor: `ScalableNumber` renamed to `QuantityValue`
+  - (!) refactor: Output stream operators moved to the `units/quantity_io.h` header file
   - refactor: basic concepts, `quantity` and `quantity_cast` refactored
   - refactor: `abs()` definition refactored to be more explicit about the return type
   - feat: interoperability with `std::chrono::duration` and other units libraries

docs/framework/text_output.rst

@@ -13,6 +13,11 @@ also tries really hard to print any quantity in the most user friendly way.
 Output Streams
 --------------
 
+.. tip::
+    
+    The streaming support is provided via the ``<units/quantity_io.h>`` header
+    file.
+
 The easiest way to print a quantity is to provide its object to the output
 stream::
 

docs/quick_start.rst

@@ -40,6 +40,7 @@ of basic library features::
     #include <units/physical/si/derived/speed.h>
     #include <units/physical/si/international/derived/speed.h>
     #include <units/format.h>
+    #include <units/quantity_io.h>
     #include <iostream>
 
     using namespace units::physical;