Add a set of contributing guidelines
Add a set of contributing guidelines

Heavily inspired by the Rust guidelines
(https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md).

maximedenes/coq,ejgallego/coq,letouzey/coq-wip,ppedrot/coq,gares/coq,SkySkimmer/coq,ppedrot/coq,ejgallego/coq,SkySkimmer/coq,JasonGross/coq,letouzey/coq-wip,silene/coq,Matafou/coq,silene/coq,Zimmi48/coq,ppedrot/coq,coq/coq,SkySkimmer/coq,Matafou/coq,gares/coq,ppedrot/coq,ppedrot/coq,maximedenes/coq,JasonGross/coq,SkySkimmer/coq,Matafou/coq,ejgallego/coq,coq/coq,letouzey/coq-wip,coq/coq,maximedenes/coq,gares/coq,JasonGross/coq,letouzey/coq-wip,Zimmi48/coq,letouzey/coq-wip,Zimmi48/coq,maximedenes/coq,gares/coq,Zimmi48/coq,ejgallego/coq,coq/coq,maximedenes/coq,JasonGross/coq,silene/coq,gares/coq