Implement always() - similar to "finally" in a "try..catch..finally" block

Implement `always()` and add to README.md.

`always` is similar to `finally` in a `try..catch..finally` block. It attaches an `onCompleted` callback that is executed when the Promise is resolved or rejected.

_NOTE:_ The specified callback does not affect the resulting Promise's outcome; any return value is ignored and any Error is rethrown.
