message: docs(contributing): add link to easy issues
  Fill the missing link to easy issues in the CONTRIBUTING.md
diff: # Contributing to Hyper
  -You want to contribute? You're awesome! Don't know where to start? Check the [list of easy issues][].
  +You want to contribute? You're awesome! Don't know where to start? Check the [list of easy issues](https://github.com/hyperium/hyper/issues?q=is%3Aopen+is%3Aissue+label%3AE-easy).
  [easy tag]: https://github.com/hyperium/hyper/issues?q=label%3AE-easy+is%3Aopen
  