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### _Why are the changes needed?_
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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>
