Mention the mkdocs.yml file for adding new documents

and move the complicated discussion about branches lower down,
hopefully most won't need to know

Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au>

@fredlf @jamtur01 @ostezer
Suggestion:

Docker docs use the `docs` branch to release [http://docs.docker.com](http://docs.docker.com)
with Docker code releases.