Replace RPC Backend text in README

Review https://review.openstack.org/#/c/192154/ removed support for RPC backends
other than RabbitMQ, but we should still document how to disable rabbit.

Change-Id: I1fd64b5f02573c58d7b0d1005c39a22c459a09a5
diff --git a/README.md b/README.md
index ebcb018..750190b 100644
--- a/README.md
+++ b/README.md
@@ -115,6 +115,16 @@
 
 `mysql` is the default database.
 
+# RPC Backend
+
+Support for a RabbitMQ RPC backend is included. Additional RPC backends may
+be available via external plugins.  Enabling or disabling RabbitMQ is handled
+via the usual service functions and ``ENABLED_SERVICES``.
+
+Example disabling RabbitMQ in ``local.conf``:
+
+    disable_service rabbit
+
 # Apache Frontend
 
 Apache web server can be enabled for wsgi services that support being deployed