[Repo]: add a CONTRIBUTING.md

[SkyeYoung]

Hi, maybe we should add a CONTRIBUTING.md?

[juzhiyuan]

Just thought about 3 views:

1. Maintainer
2. Contributor
3. Copy one from apache/apisix directly, but may be not suitable 100% for this project.

[aarishshahmohsin]

Can I be assigned this issue?