Add a warning to the third party CI doc

The third party ci doc is out of date and not currently working. It
needs help. Point to our spec that provides guidance on how to apply
that help as well as poitning to Zuul's documentation for setting up
Zuul.

Change-Id: I7365185f15f6f2cb2e3a9f6757897e793de98181

doc/source/third_party_ci.rst

@@ -1,6 +1,18 @@
 OpenStack Third-Party CI
 ========================
 
+.. warning::
+    The documentation here has been maintained by Third Party CI operators.
+    Unfortunately over time it has grown old and is no longer up to date.
+    If you are running a Third Party CI system and would like to help produce
+    an up to date document, this
+    `spec <https://specs.openstack.org/openstack-infra/infra-specs/specs/zuulv3-3rd-party-ci.html>`_
+    aims to provide guidance on how to do that.
+
+    As an alternative you may try to use Zuul's `documentation
+    <https://zuul-ci.org/docs/zuul/admin/index.html>`_ to spin up a working
+    CI system.
+
 These instructions provide a **Third Party Testing** solution using the
 same tools and scripts used by the OpenStack Infrastructure 'Jenkins' CI
 system.