docs(MIGRATION.md): added section about the new default scheduling behavior
docs(MIGRATION.md): added section about the new default scheduling behavior

fixes #1855

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