Documentation

Hello @weaverryan !

I hope you are doing fine ! :sunflower:

We are trying to uniform documentation on our public repositories adding code of conducts and contributing guidelines (you can check Snappy repository as an example [CODE_OF_CONDUCT.md](https://github.com/KnpLabs/snappy/blob/master/CODE_OF_CONDUCT.md) and [CONTRIBUTING.md](https://github.com/KnpLabs/snappy/blob/master/CONTRIBUTING.md)).

Do you mind if we create a PR to add them on this repository as well (the contributing guidelines will be more generic than Snappy's ones)?

Have a nice day !
Eve

cc @akovalyov @NicolasNSSM