Propose to create a CONTRIBUTING.md file in the root directory of the project

[Benedict Jin <as...@apache.org>]

Hello everyone,

I propose to create a CONTRIBUTING.md file in the root directory of the project. Then, when the user tries to create the first Pull Request, Github will prompt the user with a pop-up window so that the user can quickly link to the relevant document. The content of the CONTRIBUTING.md file could be a http://iotdb.apache.org/Development/HowToCommit.html link, or the mirror of the docs/Development/HowToCommit.md. FYI, [Setting guidelines for repository contributors](https://docs.github.com/en/free-pro-team@latest/github/building-a-strong-community/setting-guidelines-for-repository-contributors).

Regards,
Benedict Jin