feat: contribution guideline

[ppeeou]

There is no contribution guideline yet, so we'd better make it.

example https://github.com/mattermost/mattermost-server/blob/master/CONTRIBUTING.md https://github.com/firebase/firebase-js-sdk/blob/master/CONTRIBUTING.md https://github.com/yorkie-team/yorkie/blob/main/CONTRIBUTING.md https://reactjs.org/docs/how-to-contribute.html