docs(README): add browser test coverage matrix
docs(README): add browser test coverage matrix

relates to #998

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