Add some docs on target version numbers

This is advice copied from oslo-incubator's dispatcher.py.

Change-Id: Ib0fe7e4734eaefe177d022389df75f7bf8ee5e6a

doc/source/target.rst

@@ -5,3 +5,50 @@ Target
 .. currentmodule:: oslo.messaging
 
 .. autoclass:: Target
+
+===============
+Target Versions
+===============
+
+Target version numbers take the form Major.Minor. For a given message with
+version X.Y, the server must be marked as able to handle messages of version
+A.B, where A == X and  B >= Y.
+
+The Major version number should be incremented for an almost completely new
+API. The Minor version number would be incremented for backwards compatible
+changes to an existing API.  A backwards compatible change could be something
+like adding a new method, adding an argument to an existing method (but not
+requiring it), or changing the type for an existing argument (but still
+handling the old type as well).
+
+If no version is specified it defaults to '1.0'.
+
+In the case of RPC, if you wish to allow your server interfaces to evolve such
+that clients do not need to be updated in lockstep with the server, you should
+take care to implement the server changes in a backwards compatible and have
+the clients specify which interface version they require for each method.
+
+Adding a new method to an endpoint is a backwards compatible change and the
+version attribute of the endpoint's target should be bumped from X.Y to X.Y+1.
+On the client side, the new RPC invocation should have a specific version
+specified to indicate the minimum API version that must be implemented for the
+method to be supported.  For example::
+
+    def get_host_uptime(self, ctxt, host):
+        cctxt = self.client.prepare(server=host, version='1.1')
+        return cctxt.call(ctxt, 'get_host_uptime')
+
+In this case, version '1.1' is the first version that supported the
+get_host_uptime() method.
+
+Adding a new parameter to an RPC method can be made backwards compatible. The
+endpoint version on the server side should be bumped. The implementation of
+the method must not expect the parameter to be present.::
+
+    def some_remote_method(self, arg1, arg2, newarg=None):
+        # The code needs to deal with newarg=None for cases
+        # where an older client sends a message without it.
+        pass
+
+On the client side, the same changes should be made as in example 1. The
+minimum version that supports the new parameter should be specified.