Add instructions for manual deploy (#501)

README.rst

@@ -234,4 +234,10 @@ Releasing
 2. Publish a new release and tag ``vX.Y.Z``, using autogenerated changelog
 3. Watch for deployment notifications in the Mozilla ``#kinto-standup`` Slack channel.
 
+In order to deploy manually (eg. to production):
+
+1. Go to [deployment workflow page](https://github.com/mozilla-sre-deploy/deploy-remote-settings/actions/workflows/gcp.yaml)
+2. Click on `Run workflow`
+3. Pick `Branch=main`, `Environment=prod`, `ref=vX.Y.Z`, and click `Run workflow`
+
 See `Environments <https://remote-settings.readthedocs.io/en/latest/getting-started.html#environments>`_ section for more details about deployments.