docs: Include readme in sphinx documentation

This allows us to avoid having to duplicate any information in the
readme into another documentation file.

Signed-off-by: Paul Barker <pbarker@toganlabs.com>

docs/index.rst

@@ -11,6 +11,7 @@ Contents:
 .. toctree::
    :maxdepth: 2
 
+   readme
    layer-contents
    extra-build-config
    extra-apps

docs/readme.md

@@ -0,0 +1 @@
+../README.md