docs(contribution-guidelines): add coverity build configuration section

Added a sub-section in the "Processes and Policies" chapter under
Contributor's guide on how to add new build configurations when new
source files are added to the TF-A repository. This will help the patch
contributor to update their files to get analysed by Coverity Scan.

Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>
Change-Id: I71f410a061028f89bd0e984e48e61e5935616d71
1 file changed