Doc for project dependencies in gate

When jobs share gates, Zuul has a nice feature which is to creates
reference of the Change in every involved repositories.  That was sadly
undocumented.

Reference:

James E. Blair on openstack-infra mailing list:
http://lists.openstack.org/pipermail/openstack-infra/2014-January/000605.html

Change-Id: Ia3bd640f6a90e12192af1686547a5f4146192dbe
Merge "Doc for project dependencies in gate"