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

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