KYLIN-4799 Create CONTRIBUTING.md

[joey-yoonsung]

Proposed changes

There are documents for developers on the website. Placing CONTRIBUTING.md in the root of the GitHub repository is defacto-standard but this project doesn't. I create a CONTRIBUTING.md file and make it refer to documents on the official website of Kylin. I expect this makes other developers easy to access to development guide.

I make this PR to the master branch because the CONTRIBUTING.md file should be located in root dir in this repository. I understand the document branch only takes documents for the website. If I understand wrong, please let me know.

Types of changes

What types of changes does your code introduce to Kylin?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I have create an issue on Kylin's jira, and have described the bug/feature there in detail
• Commit messages in my PR start with the related jira ID, like "KYLIN-0000 Make Kylin project open-source"
• Compiling and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• If this change need a document change, I will prepare another pr against the document branch
• Any dependent changes have been merged

Further comments

If this is a relatively large or complex change, kick off the discussion at user@kylin or dev@kylin by explaining why you chose the solution you did and what alternatives you considered, etc...

[coveralls]

Pull Request Test Coverage Report for Build 6513

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.003%) to 28.048%

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Files with Coverage Reduction	New Missed Lines	%
stream-core/src/main/java/org/apache/kylin/stream/core/storage/CheckPointStore.java	2	73.74%

Totals
Change from base Build 6502:	0.003%
Covered Lines:	26253
Relevant Lines:	93600

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💛 - Coveralls

[shaofengshi]

LGTM, thanks!