Add documentation for Kolla plugin functionality

Every now and then people ask about plugins for Kolla. This conversation
may need to be revisted in more detail soon, but till then it's worth
highlighting what we do currently support in Kolla.

Right now adding the block as shown in this patch will make the plugin
archive available in the image, but currently only Neutron has
Dockerfile directives to automatically install them if present. The same
can be added very easily for other commonly pluggable services such as
Horizon.

Change-Id: Ia52d5ccf753667c5452b19fcaf4bf5b893a59fd0

doc/image-building.rst

@@ -126,6 +126,43 @@ Then build RHEL containers:
 
 
 
+Plugin Functionality
+--------------------
+
+.. note::
+
+  The following functionality currently exists only for Neutron. Other
+  services will be made pluggable in Kolla in the near future.
+
+  Plugin functionality is available for the source build type only.
+
+Certain OpenStack services support third party plugins, e.g. Neutron's
+pluggable L2 drivers_.
+
+Kolla supports downloading pip installable archives as part of the build, which
+will then be picked up and installed in the relevant image.
+
+To instruct Kolla to use these, add a section to
+``/etc/kolla/kolla-build.conf`` in the following format:
+
+::
+
+    [<image>-plugin-<plugin-name>]
+
+Where, ``<image>`` is the image that the plugin should be installed into, and
+``<plugin-name>`` is an identifier of your choice.
+
+For example, to install the Cisco L2 plugin for Neutron into the neutron-server
+image, one would add the following block to ``/etc/kolla/kolla-build.conf``:
+
+::
+
+    [neutron-server-plugin-networking-cisco]
+    type = git
+    location = https://github.com/openstack/networking-cisco
+    reference = master
+
+
 Known issues
 ------------
 
@@ -232,3 +269,4 @@ Finally, pass them to the build script using the ``-i`` and ``-I`` flags:
 
 
 .. _DockerBug: https://github.com/docker/docker/issues/6980
+.. _drivers: https://wiki.openstack.org/wiki/Neutron#Plugins