docs(instructions): write instructions for not reading doc/ directory
docs(instructions): write instructions for not reading doc/ directory

People have been mislead to look for docs by browsing through the doc/ directory through GitHub,
like they did with RxJS 4. This directory is not meant for content to be consumed directly through
GitHub, so this README will instruct people to look at reactivex.io/rxjs instead.

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