Add Usage section.

doc/index.rst

@@ -2,6 +2,22 @@
 
 .. _string-formatting-api:
 
+Usage
+-----
+
+To use the C++ Format library, add ``format.h`` and ``format.cc`` from
+one of the `release archives <https://github.com/cppformat/cppformat/releases>`__
+or the `Git repository <https://github.com/cppformat/cppformat>`__ to your project.
+
+If you are using Visual Studio with precompiled headers, you might need to add
+the line
+
+::
+
+   #include "stdafx.h"
+
+before other includes in ``format.cc``.
+
 C++ Format Library API
 ----------------------