Docs: Add the remote tag to the badge image

Latest docutils complains about images that are not in the same repo
unless they are properly marked as `:remote:`.

Change-Id: Ic305813473eabbe1ab43a78fc767e0d80b097a07
diff --git a/README.rst b/README.rst
index bc58cc0..281516b 100644
--- a/README.rst
+++ b/README.rst
@@ -4,6 +4,7 @@
 
 .. image:: http://governance.openstack.org/badges/tempest.svg
     :target: http://governance.openstack.org/reference/tags/index.html
+    :remote:
 
 .. Change things from this point on