Complete the documentation needed to add a new IRC channel

This change compltes the documentation about how to create a new IRC channel
for a new project.

Change-Id: I284a292d653a5eb464be2d43cccb459334227c94
Closes-Bug: #1349823

doc/source/stackforge.rst

@@ -233,6 +233,18 @@ end of this file that looks like::
       branches:
         - master
 
+If you are defining a new channel, add it also in
+``modules/openstack_project/files/accessbot/channels.yaml`` file, optionally
+defining also its mask.
+The mask will be used to define the access level for IRC users who are not
+listed in that file in the ``global`` section or otherwise listed for the
+channel.
+
+For instance:
+
+  - name: new_project
+    mask: full_mask
+
 For more information about channel requirements and IRC services provided by
 the infrastructure team, visit :ref:`irc`