tut: dynapp: add keys readme

Explains these keys are only for demonstration.

Author: bradjc

examples/tutorials/dynamic-apps-and-policies/keys/README.md

@@ -0,0 +1,8 @@
+Dynamic Apps and Policies Tutorial Example Keys
+===============================================
+
+These keys are included to demonstrate how to use signing keys with Tock apps.
+
+These keys are ENTIRELY for DEMONSTRATION. These keys are NEVER to be used in
+any sort of deployed application. These keys should be considered compromised
+and shared broadly.