Add comment to top of zuul.yaml pointing to other files

Zuul v3 config is spread across a bunch of repos, so figuring out where
somethig is defined may be daunting for a newcommer. Add a comment
to the top of the file pointing people at other repos they should be
aware of.

Also remove self-referencing roles declaration since job repos
automatically get their own roles now.

Change-Id: Ib9f966ff96f6617e8a7ce3b1e9aa4862f9e12a75

zuul.yaml

@@ -1,9 +1,12 @@
+# Shared jobs specific to the OpenStack Project
+# imports jobs from:
+#    https://git.openstack.org/cgit/openstack-infra/project-config
+#    https://git.openstack.org/cgit/openstack-infra/zuul-jobs
+
 - job:
     name: openstack-doc-build
     parent: tox-docs
     run: playbooks/tox/docs
-    roles:
-      - zuul: openstack-infra/openstack-zuul-jobs
 
 - job:
     name: tox-py35-on-zuul