Mention matrix_synapse_allow_public_rooms_over_federation in the docs

Fixes #502 (Github Issue)

docs/configuring-playbook-federation.md

@@ -17,6 +17,17 @@ matrix_synapse_federation_domain_whitelist:
 If you wish to disable federation, you can do that with an empty list (`[]`), or better yet by completely disabling federation (see below).
 
 
+## Exposing the room directory over federation
+
+By default, your server's public rooms directory is not exposed to other servers via federation.
+
+If you wish to expose it, add this to your configuration file (`inventory/host_vars/matrix.<your-domain>/vars.yml`):
+
+```yaml
+matrix_synapse_allow_public_rooms_over_federation: true
+```
+
+
 ## Disabling federation
 
 To completely disable federation, isolating your server from the rest of the Matrix network, add this to your configuration file (`inventory/host_vars/matrix.<your-domain>/vars.yml`):