Add warning to custom-templates.yaml: Newton only

As per https://bugzilla.redhat.com/show_bug.cgi?id=1439223, this template for RH-OSP10j, 
was run in RH-OSP11 but the base definition of the compute role changed in 11. Added 
warning to top of custom-templates.yaml that this file is for 10 (Newton) only and suggested 
process to create a new version for those using 11 (Mitaka). 

FYI: for 11/Mitaka HCI will be productized and thus official documentation will be available, 
not an update RA.

custom-templates/custom-roles.yaml

@@ -1,3 +1,13 @@
+# Valid for Newton/RH-OSP10 ONLY
+# 
+# If using Mitaka/RH-OSP11, then:
+# 
+#  cp /usr/share/openstack-tripleo-heat-templates/roles_data.yaml ~/custom-templates/custom-roles.yaml
+#  vi ~/custom-templates/custom-roles.yaml 
+# 
+# and then edit Mitaka versin to compose a OsdCompute by copying Compute 
+# Then add "- OS::TripleO::Services::CephOSD"
+# 
 # Specifies which roles (groups of nodes) will be deployed
 # Note this is used as an input to the various *.j2.yaml
 # jinja2 templates, so that they are converted into *.yaml