Workflow documentation is now in infra-manual

Replace URLs for workflow documentation to appropriate parts of the
OpenStack Project Infrastructure Manual.

Change-Id: I5637374ac8b957e948c2345a883636198cf15e9b

CONTRIBUTING.rst

@@ -1,7 +1,7 @@
 Changes should be submitted for review via the Gerrit tool,
 following the workflow documented at:
 
-   http://wiki.openstack.org/GerritWorkflow
+   http://docs.openstack.org/infra/manual/developers.html#development-workflow
 
 Pull requests submitted through GitHub will be ignored.