chore(CHANGELOG): add link to lettable operators doc
chore(CHANGELOG): add link to lettable operators doc

ReactiveX/RxJS,ReactiveX/RxJS,jayphelps/rxjs,IgorMinar/RxJS,ReactiveX/rxjs,ReactiveX/rxjs,martinsik/rxjs,trxcllnt/RxJS,jayphelps/rxjs,kwonoj/RxJS,kwonoj/RxJS,martinsik/rxjs,ReactiveX/RxJS,benlesh/RxJS,martinsik/rxjs,trxcllnt/RxJS,ReactiveX/rxjs,benlesh/RxJS,ReactiveX/rxjs,benlesh/RxJS,Brocco/RxJS,trxcllnt/RxJS,jayphelps/rxjs,martinsik/rxjs,jayphelps/rxjs,kwonoj/RxJS,trxcllnt/RxJS,Brocco/RxJS,jayphelps/rxjs,benlesh/RxJS,IgorMinar/RxJS,ReactiveX/RxJS,kwonoj/RxJS,Brocco/RxJS,Brocco/RxJS