Disabling external-security example as it doesn't work on ML9

Author: Rob Rudin

examples/sample-project/src/main/ml-config/security/external-security/README.md

@@ -1,2 +1,5 @@
 See [the MarkLogic docs](http://docs.marklogic.com/REST/POST/manage/v2/external-security) for what an external 
-security JSON/XML file can contain.
+security JSON/XML file can contain.
+
+This example no longer works on ML9, as a keystore is required. ml-gradle will soon
+have support for this new feature in ML9.