docs(contributing): various improvements

 - Warn contributors that they need to register their email address in
   their Gerrit profile. Not doing so causes errors at patch submission
   and is a recurrent question on the mailing list.

 - Add some links where useful.

 - Remove confusing CGit link to TF-A source code. In the context of
   setting up a local copy of the repo for contributing patches,
   developers should rather clone it through Gerrit and this is best
   covered by the "Getting the TF-A Source" section of TF-A
   documentation.

 - Add references to the OpenCI documentation, which has a lot more
   details on some of the topics we briefly cover in the contribution
   guidelines.

 - Encourage the user to use the 'git review' command for patch
   submission, inline with OpenCI documentation instructions. This
   automatically sorts out which Gerrit server to push to and against
   which repo branch (thanks to the '.gitreview' configuration file in
   TF-A root directory).

 - Elaborate the Coverity Scan section.

Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Change-Id: I1131662d8bc3502967b269a599869ea130897efb
1 file changed