message: Add missing `stretch` link in `README`
diff: @@ -163,6 +163,7 @@ mutability at all! There is not a single `Rc`, `RefCell`, or similar used
  directly in the library.
  [this pull request]: https://github.com/hecrj/coffee/pull/35
  +[`stretch`]: https://github.com/vislyhq/stretch
  [Coffee 0.3.0]: https://github.com/hecrj/coffee/releases/tag/0.3.0
  ## Contributing / Feedback
  