Add project-config reviewing guidelines

As discussed at the OpenStack summit [1], document a few guidelines on
reviewing project-config changes.

The list is not exhaustive.

[1]
https://etherpad.openstack.org/p/newton-infra-proposal-jobs

Change-Id: I894782f0d7e69bc5385b30a940e21cfb49dcbe08

REVIEWING.rst

@@ -0,0 +1,68 @@
+========================
+Reviewing project-config
+========================
+
+The following are notes for reviewers on how to review changes for
+project-config. These notes are not exhaustive, they cover a few
+caveats that especially core reviewers should be aware of.
+
+Additional reviews
+==================
+
+For changes to projects, there should be support by the project team
+that is affected. A couple of projects have listed infra liasons at
+https://wiki.openstack.org/wiki/CrossProjectLiaisons#Infra . Changes
+should only merge when at least one of the infra liasons has given a
++1. For not-listed projects, there are no rules but it might be good
+to ask for review by PTL or cores of the repo depending on the change.
+
+Voting jobs
+===========
+
+There should be no non-voting jobs in the gate queue. Voting jobs
+should be in both check and gate queues.
+
+New repo creations
+==================
+
+Check the following:
+
+* If publishing to PyPI is set up: Check that
+  https://pypi.python.org/pypi/PROJECT exists and is set up for
+  openstackci.
+
+* Is this a new repository for a team that is part of the Big Tent?
+  Then ask for a governance review and PTL+1.
+
+* If there's no import ("upstream" keyword) of an existing repository,
+  best check that the team really wants to start with a new empty repo
+  and has no content to import. Either is fine, it's just that an
+  import at repo creation time is easy, afterwards it only causes
+  problems.
+
+Big Tent resources
+==================
+
+Check that publishing to docs.openstack.org or specs.openstack.org is
+only enabled for projects that are in the Big Tent (mentioned in
+governance repository). Similary, translation workflow is also only
+enabled for Big Tent projects.
+
+Proposal jobs
+=============
+
+Proposal jobs run on the long running proposal slave that has access
+to the credentials that the jobs need to access Zanata and gerrit.
+Jobs running there are the exception and need to be carefully reviewed
+before approving them.
+
+Here are some points to look at:
+
+* First response should be: How else can we achieve this?
+
+* Jobs can publish artifacts also to specific places, like
+  static.openstack.org or tarballs.openstack.org, and use these from
+  other jobs.
+
+* Proposal jobs should not run arbitrary scripts from other
+  repositories or install untrusted packages.