README: add a new README file, pointing to the Documentation/

As we moved the real README file to Documentation/admin-guide/README.rst,
let's add a replacement, pointing to it, and giving the main directions
about documentation.

In the future, perhaps it would be worth to move the contents
of Documentation/00-Index into this README.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>