Add contributing.md

[weshayutin]

Why the changes were made

OADP did not yet have a contributing guide.

How to test the changes made

READ :)

[weshayutin]

contributing - #1586
code of conduct - #1576

[kaovilai]

/override ci/prow/4.17-ci-index

This is not relevant to .md

[openshift-ci]

@kaovilai: Overrode contexts on behalf of kaovilai: ci/prow/4.17-ci-index

In response to this:

/override ci/prow/4.17-ci-index

This is not relevant to .md

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[kaovilai]

/lgtm

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kaovilai, shubham-pampattiwar, weshayutin

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• OWNERS [kaovilai,shubham-pampattiwar]

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[openshift-ci]

@weshayutin: all tests passed!

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