Added missing item from Changelog

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)

docs/sources/release-notes.md

@@ -31,6 +31,8 @@ Specifically, changes were made to:
 
 * Fixed escaping `$` for environment variables.
 * Fixed issue with lowercase `onbuild` instruction in a `Dockerfile`.
+* Restricted environment variable expansion to `ENV`, `ADD`, `COPY`, `WORKDIR`,
+`EXPOSE`, `VOLUME`, and `USER`
 
 ##Version 1.3.0