feat: add contributing.md

[healthjyk]

What type of PR is this?

/kind feature

What this PR does / why we need it:

add contributing.md

Which issue(s) this PR fixes:

Fixes #514

[coveralls]

Pull Request Test Coverage Report for Build 6181983044

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 7 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.07%) to 74.122%

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Files with Coverage Reduction	New Missed Lines	%
pkg/engine/operation/parser/parser.go	3	79.27%
pkg/engine/operation/parser/spec_parser.go	4	56.1%

Totals
Change from base Build 6181960614:	0.07%
Covered Lines:	6267
Relevant Lines:	8455

---

💛 - Coveralls

[SparkYuan]

lgtm