Add information about test databases in contributing guide
Add information about test databases in contributing guide

Chocobozzz/PeerTube,Chocobozzz/PeerTube,Chocobozzz/PeerTube,Green-Star/PeerTube,Green-Star/PeerTube,Chocobozzz/PeerTube,Green-Star/PeerTube,Green-Star/PeerTube