Update README with some basic contribution guidelines.
Update README with some basic contribution guidelines.

zongkelong/rivets,QAPInt/rivets,GerHobbelt/rivets,GerHobbelt/rivets,nopnop/rivets,jccazeaux/rivets,mikeric/rivets,kangax/rivets,zongkelong/rivets,mikeric/rivets,GerHobbelt/rivets,re-clone/rivets,MishaMykhalyuk/rivets,re-clone/rivets,altmind/rivets,moneyadviceservice/rivets,QAPInt/rivets,nopnop/rivets,altmind/rivets,benderTheCrime/tiny-rivets,zongkelong/rivets,MishaMykhalyuk/rivets,jccazeaux/rivets,npmcomponent/mikeric-rivets,jccazeaux/rivets,re-clone/rivets,mikeric/rivets,MishaMykhalyuk/rivets,nopnop/rivets,QAPInt/rivets,altmind/rivets