Changed collaborator administrators
Changed collaborator administrators

* Denommus is no longer active (he is of course welcome back!)
* mitchmindtree is added as reserve, in case bvssvni is unavailable (volunteers to be reserves are welcome)
* Added notice about PRs
mitchmindtree/piston,PistonDevelopers/piston,bvssvni/piston,bvssvni/piston,tempbottle/piston,getynge/piston,placrosse/piston,oldmanmike/piston,KevinFrans3/piston,PistonDevelopers/piston,Potpourri/piston,axelmagn/piston,caldwell/piston