Document how to expand loopabull's usage

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>

README.rst

@@ -11,6 +11,46 @@ This repository stores the ansible playbooks and their related scripts used by
 the `Fedora`_ `Infrastructure`_ team.
 
 
+Adding a new task
+-----------------
+
+Adding a new task to loopabull is quite easy.
+
+* Figure out which fedmsg topic you want to trigger off
+* Create a playbook named after the topic
+* Create a role doing the action you want to have triggered of this topic
+* Configure loopabull to listen to this topic
+* Push the new loopabull configuration
+
+For example, let's run a small/dummy python script everytime a koji build
+finished in staging.
+
+The topic we want to trigger of is thus: ``org.fedoraproject.stg.buildsys.build.state.change``,
+so we go and edit the playbook: ``playbooks/org.fedoraproject.stg.buildsys.build.state.change.yml``
+in this repository and add a `debug` role that has one variable: the message
+received via fedmsg `msg`.
+Then add a ``debug`` role in ``playbooks/roles/debug``. Make that role install
+your python script, call it with a certain environment variable and print
+the output of the script for debugging.
+
+Then, on the infrastructure's ansible repository, configure loopabull to listen
+to this topic (line 70):
+https://infrastructure.fedoraproject.org/cgit/ansible.git/tree/playbooks/groups/loopabull.yml?id=4b053dd68cc12e1823516eec481fab04f9b9166b#n70
+
+Finally, run the ansible playbook deploying this change:
+
+::
+
+    ansible-playbook /srv/web/infra/ansible/playbooks/groups/loopabull.yml -t loopabull
+
+
+.. note:: you may have to restart things on loopabull's side:
+
+    ::
+
+        systemctl restart loopabull loopabull@1 loopabull@2 loopabull@3 loopabull@4 loopabull@5
+
+
 Good Practice
 -------------