Add warning about mixing Kolla and OpenStack releases

Seen several instances of users having trouble attempting to mix and
match Kolla and OpenStack releases. Add warning that this is not
supported.

Change-Id: Ibdb9e584646599e49a698cfcfaf5f114b5b8943a

doc/quickstart.rst

@@ -446,6 +446,11 @@ In order to see all available parameters, run:
 For more information about building Kolla container images, check the detailed
 instruction in :doc:`image-building`.
 
+.. WARNING::
+    Mixing of OpenStack releases with Kolla releases (example, updating
+    kolla-build.conf to build Mitaka Keystone to be deployed with Newton Kolla) is
+    not recommended and will likely cause issues.
+
 .. _deploying-kolla:
 
 Deploying Kolla