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/index.rst b/docs/process/index.rst
index 1cb5354..37324b0 100644
--- a/docs/process/index.rst
+++ b/docs/process/index.rst
@@ -11,5 +11,6 @@
    coding-style
    coding-guidelines
    contributing
+   code-review-guidelines
    faq
    security-hardening