Add CONTRIBUTING.md (#6)

[erdaltsksn]

[coveralls]

Pull Request Test Coverage Report for Build 344399269

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 0.0%

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Totals
Change from base Build 340270277:	0.0%
Covered Lines:	0
Relevant Lines:	0

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💛 - Coveralls

[maaslalani]

Hey @erdaltsksn, thank you so much for the PR. I've added a CONTRIBUTING.md guide in #405