Edit: Thanks for your kind comments on disabling contributions. I'm aware of the possibility, this is just a meme.
Contributions in open source projects are great, needed and make the world a better place after all. Therefore they should be declined politely, optionally by giving a reason.
I hereby inform you that I will no longer accept contributions to this post /s.
While I do agree GitHub is a place where people collaborate, it's also a pretty handy place to store stuff without having to host your own. If the project doesn't invite people to contribute, don't expect a polite response forever. It's like stopping your neighbor on the street corner to tell them they should paint their house white for the thermal benefits, yeah people collaborate on the street, yeah you are right, it's their house though.