Add short README file for contrib/template-override directory

Change-Id: Ie6da4d94829126d1d8d3156ac5e608d965ffc2a6

contrib/template-override/README.rst

@@ -0,0 +1,9 @@
+contrib/template-override directory
+===================================
+
+Template override files stored in this directory will not be gated and serve as an
+example of how to extend in-tree container images for specific use cases, for example
+source build of infrastructure containers.
+
+Templates stored in this directory should have corresponding documentation in
+``doc/template-override`` to describe their use.