message: docs(README): mention lettable operators
  Add a mention of lettable operators alongside other import techniques.
diff: @@ -67,6 +67,8 @@ import { map } from 'rxjs/operator/map';
  Observable::of(1,2,3)::map(x => x + '!!!'); // etc
  ```
  +Alternatively, you can use the built-in `pipe` method on Observables. See [lettable operators](https://github.com/ReactiveX/rxjs/blob/master/doc/lettable-operators.md) for more information.
  +
  ### CommonJS via npm
  To install this library for CommonJS (CJS) usage, use the following command:
  