Doc: Restructured readme.rst

This patch converts the readme.rst, placed outside
of the documentation directory to a quick reference
page, using direct linking for hyperlinks. Doing so
would allow cgit to render it properly

Contents of the file have been moved to the index.rst.in

Change-Id: I6769465561ccaf98957af325103bef8b89b71026
Signed-off-by: Galanakis, Minos <minos.galanakis@arm.com>