Prepare for next spec dev cycle

changelogs/client_server.rst

@@ -1,3 +1,6 @@
+Unreleased changes
+==================
+
 r0.3.0
 ======
 

specification/client_server_api.rst

@@ -46,6 +46,7 @@ Other versions of this specification
 The following other versions are also available, in reverse chronological order:
 
 - `HEAD <https://matrix.org/docs/spec/client_server/unstable.html>`_: Includes all changes since the latest versioned release.
+- `r0.3.0 <https://matrix.org/docs/spec/client_server/r0.3.0.html>`_
 - `r0.2.0 <https://matrix.org/docs/spec/client_server/r0.2.0.html>`_
 - `r0.1.0 <https://matrix.org/docs/spec/client_server/r0.1.0.html>`_
 - `r0.0.1 <https://matrix.org/docs/spec/r0.0.1/client_server.html>`_