docs(README.md): clarify npm install instructions
docs(README.md): clarify npm install instructions

Clarify instructions on how to install this library using npm 3 or npm 2.

Avoids issues such as #727 from happening.
Brocco/RxJS,jayphelps/rxjs,ReactiveX/rxjs,jinroh/RxJS,RxJS-CN/RxJS-Docs-CN,Igorbek/rxjs5,Brocco/RxJS,jayphelps/rxjs,Brocco/RxJS,gluck/RxJS,adrianomelo/RxJS-1,trxcllnt/RxJS,justinwoo/RxJS,mattpodwysocki/RxJS,jinroh/RxJS,martinsik/rxjs,ReactiveX/RxJS,IgorMinar/RxJS,robwormald/RxJS,saneyuki/RxJS,elliotschi/rxjs,jayphelps/rxjs,SangKa/RxJS-Docs-CN,staltz/RxJSNext,figueredo/RxJS,mattpodwysocki/RxJS,staltz/RxJSNext,mattpodwysocki/RxJS,Brocco/RxJS,figueredo/RxJS,ReactiveX/rxjs,robwormald/RxJS,SangKa/RxJS-Docs-CN,kwonoj/RxJS,elliotschi/rxjs,SangKa/RxJS-Docs-CN,kwonoj/RxJS,staltz/RxJSNext,benlesh/RxJS,adrianomelo/RxJS-1,gluck/RxJS,IgorMinar/RxJS,justinwoo/RxJS,kwonoj/RxJS,staltz/RxJSNext,robwormald/RxJS,robwormald/RxJS,benlesh/RxJS,justinwoo/RxJS,jinroh/RxJS,Igorbek/rxjs5,adrianomelo/RxJS-1,trxcllnt/RxJS,ReactiveX/RxJS,RxJS-CN/RxJS-Docs-CN,adrianomelo/RxJSNext,mattpodwysocki/RxJS,martinsik/rxjs,figueredo/RxJS,jayphelps/rxjs,martinsik/rxjs,jadbox/RxJS,jadbox/RxJS,ReactiveX/RxJS,Igorbek/rxjs5,figueredo/RxJS,jadbox/RxJS,justinwoo/RxJS,trxcllnt/RxJS,RxJS-CN/RxJS-Docs-CN,martinsik/rxjs,adrianomelo/RxJS-1,trxcllnt/RxJS,saneyuki/RxJS,adrianomelo/RxJSNext,RxJS-CN/RxJS-Docs-CN,benlesh/RxJS,ReactiveX/rxjs,benlesh/RxJS,adrianomelo/RxJSNext,ReactiveX/rxjs,Igorbek/rxjs5,adrianomelo/RxJSNext,gluck/RxJS,jayphelps/rxjs,SangKa/RxJS-Docs-CN,ReactiveX/RxJS,kwonoj/RxJS,elliotschi/rxjs,jadbox/RxJS,gluck/RxJS,elliotschi/rxjs,jinroh/RxJS