Update README.rst to not this repo is deprecated.

Author: mhammond

README.rst

@@ -7,6 +7,12 @@ Run-Your-Own Firefox Sync Server
 .. image:: https://img.shields.io/docker/automated/mozilla-services/syncserver.svg?style=flat-square
    :target: https://hub.docker.com/r/mozilla/syncserver/
 
+**Note that this repository is deprecated**. It is based on Python 2, and is unlikely to
+ever be upgraded, or ever run again in production by Mozilla. We are unlikely to respond
+to requests for help using this package.
+
+Please look into https://github.com/mozilla-services/syncstorage-rs instead.
+
 This is an all-in-one package for running a self-hosted Firefox Sync server.
 It bundles the "tokenserver" project for authentication and the "syncstorage"
 project for storage, to produce a single stand-alone webapp.