Add a new example pointer to the retail-supergraph repo

Author: jonathanrainer

examples/README.md

@@ -0,0 +1,6 @@
+# Examples
+
+Apollo publishes a repo called [retail-supergraph](https://github.com/apollosolutions/retail-supergraph/blob/main/DEV.md)
+which demonstrates running a supergraph using Rover.
+
+Follow the instructions contained in the link above to run an example of a supergraph with `rover dev`.