message: clarify cargo branches for beta promotion
diff: @@ -80,6 +80,10 @@ $ git fetch rust-lang
  $ git push rust-lang rust-lang/master:rust-1.14.0
  ```
  +You'll need to temporarily disable branch protection on GitHub to push the new
  +branch. Make sure the commit in the new branch is the latest one on
  +rust-lang/rust, and not an earlier one.
  +
  In theory one day we'll do the same for rust-lang/rls, but for now we
  haven't done this yet.
  