docs(README): update observable proposal status
docs(README): update observable proposal status

Observables did not make it into ES7 (not much did) which is feature frozen. I changed the wording to indicate proposal instead. I am glad to reword that if necessary.

jayphelps/rxjs,elliotschi/rxjs,jayphelps/rxjs,benlesh/RxJS,robwormald/RxJS,gluck/RxJS,ReactiveX/RxJS,RxJS-CN/RxJS-Docs-CN,kwonoj/RxJS,adrianomelo/RxJSNext,kwonoj/RxJS,robwormald/RxJS,benlesh/RxJS,SangKa/RxJS-Docs-CN,elliotschi/rxjs,saneyuki/RxJS,ReactiveX/rxjs,SangKa/RxJS-Docs-CN,staltz/RxJSNext,RxJS-CN/RxJS-Docs-CN,kwonoj/RxJS,robwormald/RxJS,staltz/RxJSNext,adrianomelo/RxJSNext,jayphelps/rxjs,adrianomelo/RxJSNext,benlesh/RxJS,staltz/RxJSNext,adrianomelo/RxJS-1,Brocco/RxJS,jayphelps/rxjs,trxcllnt/RxJS,Brocco/RxJS,martinsik/rxjs,SangKa/RxJS-Docs-CN,adrianomelo/RxJSNext,adrianomelo/RxJS-1,Brocco/RxJS,IgorMinar/RxJS,figueredo/RxJS,ReactiveX/rxjs,trxcllnt/RxJS,RxJS-CN/RxJS-Docs-CN,jayphelps/rxjs,Igorbek/rxjs5,figueredo/RxJS,RxJS-CN/RxJS-Docs-CN,ReactiveX/RxJS,Igorbek/rxjs5,ReactiveX/RxJS,adrianomelo/RxJS-1,IgorMinar/RxJS,figueredo/RxJS,martinsik/rxjs,figueredo/RxJS,gluck/RxJS,staltz/RxJSNext,elliotschi/rxjs,elliotschi/rxjs,trxcllnt/RxJS,saneyuki/RxJS,adrianomelo/RxJS-1,Igorbek/rxjs5,gluck/RxJS,martinsik/rxjs,ReactiveX/rxjs,kwonoj/RxJS,trxcllnt/RxJS,martinsik/rxjs,benlesh/RxJS,robwormald/RxJS,ReactiveX/rxjs,ReactiveX/RxJS,Igorbek/rxjs5,Brocco/RxJS,gluck/RxJS,SangKa/RxJS-Docs-CN