docs: add code reviews to contributor guide

(cherry picked from kolla commit abc4f7c124265f9e7b72d448b2ef18d6d4e84888)

Change-Id: I7e51a76287cd0f5a22d11dc3af9284b556d06eca

doc/source/contributor/code-review.rst

+============
+Code Reviews
+============
+
+All Kolla code must be reviewed and approved before it can be merged. Anyone
+with a Gerrit account is able to provide a review. Two labels are available to
+everyone:
+
+* +1: Approve
+* -1: Changes requested
+
+It is also possible to leave comments without a label. In general, a review
+with comments is more valuable. Comments are especially important for a
+negative review. Prefer quality of reviews over quantity.
+
+You can watch specific patches in Gerrit via *Settings -> Watched Projects*.
+The volume of emails is not too large if you subscribe to *New Changes* only.
+If you do not have much time available for reviewing, consider reviewing
+patches in an area that is important to you or that you understand well.
+
+Core reviewers
+==============
+
+Core reviewers have additional labels available to them.
+
+* +2: Approve
+* -2: Do not merge
+* Workflow +1: Approve and ready for merge
+
+Zuul requires one +2 and one workflow +1, as well as a passing check, in order
+for a patch to proceed to the gate. The Kolla team generally requires two +2s
+before a workflow +1 may be added. We also have some non-voting Zuul jobs which
+will not block a check, but should be investigated if they are failing.
+
+Core reviewers may still use +1 to indicate approval if they are not confident
+enough about a particular patch to use +2.
+
+The Kolla core reviewers have the same rights of access to stable branches, so
+always check the branch for a review, and use extra care with stable branches.
+
+Becoming a core reviewer
+------------------------
+
+There are no strict rules for becoming a core reviewer. Join the community,
+review some patches, and demonstrate responsibility, understanding & care. If
+you are interested in joining the core team, ask the PTL or another core
+reviewer how to get there.

doc/source/contributor/index.rst

@@ -17,6 +17,7 @@ We welcome everyone to join our project!
    release-notes
    vagrant-dev-env
    running-tests
+   code-review
    kolla-for-openstack-development
    bug-triage
    ptl-guide