docs: Subscribing events using registry decorator

We supports using registry decorators to subscribe events, this
patch add it in our guide.

TrivialFix

Change-Id: Iaf3a6712aecd063b8cc272b31e374af35347c03f

doc/source/contributor/callbacks.rst

@@ -456,6 +456,17 @@ The output is:
   Notifying...
 
 
+Subscribing events using registry decorator
+-------------------------------------------
+
+Now neutron-lib supports using registry decorators to subscribe events.
+There are two decorators ``has_registry_receivers``, which sets up the
+class ``__new__`` method to subscribe the bound method in the callback registry
+after object instantiation. ``receives`` use to decorate callback method
+which must defines the resource and events.
+Any class use ``receives`` must be decorated with ``has_registry_receivers``.
+
+
 Testing with callbacks
 ----------------------