message: chore(CHANGELOG): add link to lettable operators doc
diff: <a name="5.5.0-beta.0"></a>
  # [5.5.0-beta.0](https://github.com/ReactiveX/RxJS/compare/5.4.3...5.5.0-beta.0) (2017-09-22)
  +**Important! Checkout the explanation of the new [lettable operators features here](doc/lettable-operators.md)**
  ### Bug Fixes
  