docs: code review guidelines

docs: code review guidelines

Document the code review process in TF-A.
Specifically:

 * Give an overview of code review and best practices.
 * Give guidelines for the participants in code review.
 * Outline responsibilities of each type of participant.
 * Explain the Gerrit labels used in the review process.

Change-Id: I519ca4b2859601a7b897706e310f149a0c92e390
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Signed-off-by: David Horstmann <david.horstmann@arm.com>