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>
diff --git a/docs/process/contributing.rst b/docs/process/contributing.rst
index 0b3b848..15c2b45 100644
--- a/docs/process/contributing.rst
+++ b/docs/process/contributing.rst
@@ -90,6 +90,8 @@
       (and nothing else) in the last commit of the series. Otherwise, include
       the documentation changes within the single commit.
 
+.. _copyright-license-guidance:
+
 -  Ensure that each changed file has the correct copyright and license
    information. Files that entirely consist of contributions to this project
    should have a copyright notice and BSD-3-Clause SPDX license identifier of