Change README from rst to md.

This is necessary to have a proper project
description with build instructions on
the gitorious page.
Other websites use rst, but it seems that
gitorious only supports md.

Change-Id: I5a987bca9a61e025f0ba81bb93ad2b9265a119b2
Reviewed-by: Andras Becsi <andras.becsi@digia.com>