Update CONTRIBUTING.rst.
Update CONTRIBUTING.rst.

Add some info about plugins.
mmetak/streamlink,gtmanfred/livestreamer,intact/livestreamer,Feverqwe/livestreamer,hmit/livestreamer,blxd/livestreamer,okaywit/livestreamer,beardypig/streamlink,derrod/livestreamer,fishscene/streamlink,chrippa/livestreamer,back-to/streamlink,melmorabity/streamlink,sbstp/streamlink,programming086/livestreamer,gravyboat/streamlink,wolftankk/livestreamer,wlerin/streamlink,flijloku/livestreamer,chrippa/livestreamer,streamlink/streamlink,Masaz-/livestreamer,Saturn/livestreamer,jtsymon/livestreamer,lyhiving/livestreamer,blxd/livestreamer,breunigs/livestreamer,charmander/livestreamer,sbstp/streamlink,flijloku/livestreamer,gtmanfred/livestreamer,okaywit/livestreamer,gravyboat/streamlink,chrisnicholls/livestreamer,wlerin/streamlink,streamlink/streamlink,Klaudit/livestreamer,javiercantero/streamlink,beardypig/streamlink,chhe/streamlink,caorong/livestreamer,asermax/livestreamer,lyhiving/livestreamer,fishscene/streamlink,charmander/livestreamer,hmit/livestreamer,ethanhlc/streamlink,jtsymon/livestreamer,programming086/livestreamer,derrod/livestreamer,Dobatymo/livestreamer,Dobatymo/livestreamer,melmorabity/streamlink,javiercantero/streamlink,Klaudit/livestreamer,ethanhlc/streamlink,mmetak/streamlink,chhe/livestreamer,chhe/streamlink,breunigs/livestreamer,back-to/streamlink,Feverqwe/livestreamer,wolftankk/livestreamer,bastimeyer/streamlink,Masaz-/livestreamer,chhe/livestreamer,bastimeyer/streamlink,caorong/livestreamer,intact/livestreamer,Saturn/livestreamer