Write contribution guidelines

Create a `CONTRIBUTING.md` file that outlines the contribution process

Good examples:
- [OpenZeppelin](https://github.com/OpenZeppelin/zeppelin-solidity/blob/master/CONTRIBUTING.md)
- [Gitcoin](https://github.com/gitcoinco/web/blob/master/docs/CONTRIBUTING.md)
- [Atom](https://github.com/atom/atom/blob/master/CONTRIBUTING.md#your-first-code-contribution)