Clarify the purpose of this repository

Over time, we encounter people who have reused contents from this
repository for their own systems, and would like to collaborate with
us to make changes which ease their external use case. While
extremely flattering, that's not the intent of our publishing of the
files here, and accepting changes to it which support use cases
other than our own represents additional risk to our production
services.

Update the introduction and contributing sections of the README file
in order to explain that such changes will be rejected by our
reviewers.

Change-Id: Ib46160b261508a6d530136777b6472dc34218b90

README.rst

@@ -18,6 +18,15 @@ Although the act of running puppet on these hosts is managed by
 Ansible, the actual core of their orchestration lives in ``manifests``
 and ``modules``.
 
+The files in this repository are provided as an opinionated example
+service deployment, and to allow the OpenDev Collaboratory to use public
+software development workflows in order to coordinate changes and
+improvements to the systems it runs. This repository is not intended as
+a reconsumable project on its own, and anyone wishing to adjust it to
+suit their own needs should do so with a fork. The system-config
+reviewers are unable to evaluate and support use cases for the contents
+here other than their own.
+
 Testing
 -------
 
@@ -60,7 +69,13 @@ automatically tested, reviewed by humans and, once accepted, deployed
 automatically.
 
 Bug fixes or modifications to existing code are great places to start,
-and you will see the results of your changes in CI testing.
+and you will see the results of your changes in CI testing. Please
+remember that this repository consists of configuration and
+orchestration for OpenDev Collaboratory production systems, so
+contributions to it will be evaluated on the basis of whether they're
+useful or applicable to OpenDev's services. Changes intended to make the
+contents more easily reusable outside OpenDev itself are not in scope,
+and so will be rejected by reviewers.
 
 You can develop all the playbooks, roles, containers and testing
 required for a new service just by uploading a change.  Using a