Combine all of the contribution guide together

[lijiarui]

As talked with @iamrajiv https://github.com/wechaty/wechaty.js.org/pull/917

I think wechaty should have only one contributing file, it should be: http://wechaty.js.org/docs/contributing/

We should merge wechaty/openapi#16 and https://github.com/wechaty/wechaty/blob/master/CONTRIBUTING.md to http://wechaty.js.org/docs/contributing/ together, and all of the start contributing link is the unified link: http://wechaty.js.org/docs/contributing/

I suggest [Introduction] team improves contributing page in the next period, how do you think so? @Rohitesh-Kumar-Jain

[huan]

I agree that all the CONTRIBUTING.md files in our wechaty projects' repo should be simply linked to https://wechaty.js.org/docs/contributing/ , if anyone wants to create it.

It's also acceptable to not have CONTRIBUTING.md at all, by adding a ## Contributing section in the README.md file.