Add notice to import from code.cloudfoundry.org

* also fix links in README

[#121932033]

Signed-off-by: James Myers <jmyers@pivotal.io>

README.md

@@ -1,4 +1,7 @@
 # Executor
+
+**Note**: This repository should be imported as `code.cloudfoundry.org/executor`.
+
 Let me run that for you
 
 ####Learn more about Diego and its components at [diego-design-notes](https://github.com/cloudfoundry/diego-design-notes)