docs(manual): write content for observer.md
docs(manual): write content for observer.md

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