Create CODE_OF_CONDUCT.md

[MichaelMarner]

So I'm interested in helping create a friendly, inclusive development community around Flutter. I think it's a good idea to have a code of conduct for contributors.

[coveralls]

Pull Request Test Coverage Report for Build 100

• 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 91.667%

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Totals
Change from base Build 97:	0.0%
Covered Lines:	88
Relevant Lines:	96

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💛 - Coveralls