<!--
Thanks for sending a pull request!

Here are some tips for you:
  1. If this is your first time, please read our contributor guidelines: https://kyuubi.readthedocs.io/en/latest/community/contributions.html
  2. If the PR is related to an issue in https://github.com/apache/incubator-kyuubi/issues, add '[KYUUBI #XXXX]' in your PR title, e.g., '[KYUUBI #XXXX] Your PR title ...'.
  3. If the PR is unfinished, add '[WIP]' in your PR title, e.g., '[WIP][KYUUBI #XXXX] Your PR title ...'.
-->

### _Why are the changes needed?_
<!--
Please clarify why the changes are needed. For instance,
  1. If you add a feature, you can talk about the use case of it.
  2. If you fix a bug, you can clarify why it is a bug.
-->

Add CONTRIBUTING.md to list COMMUNITY resources that I contributor can use
delete contributions and improvement-proposal in main repo

### _How was this patch tested?_
- [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible

- [ ] Add screenshots for manual tests if appropriate

- [x] [Run test](https://kyuubi.apache.org/docs/latest/develop_tools/testing.html#running-tests

) locally before make a pull request

Closes #2004 from yaooqinn/CONTRIBUTING.

Closes #2004

ddbcd521 [Kent Yao] CONTRIBUTING
f81c14b3 [Kent Yao] Merge branch 'master' of github.com:apache/incubator-kyuubi into CONTRIBUTING
3bdbdc9b [Kent Yao] CONTRIBUTING
574ceff2 [Kent Yao] CONTRIBUTING
f0a6b4c4 [Kent Yao] CONTRIBUTING
c55c6a02 [Kent Yao] CONTRIBUTING
Authored-by: Kent Yao <yao@apache.org>
Signed-off-by: ulysses-you <ulyssesyou@apache.org>