docs: adding contribution guidelines and dco

[mike-hoang]

Signed-off-by: Michael Hoang mhoang@redhat.com

What does this PR do / why we need it:

Outlines the contribution guidelines for the devfile-web repo.

Which issue(s) this PR fixes:

devfile/api#772, devfile/api#878

PR acceptance criteria:

• Unit Tests
• E2E Tests
• Documentation

How to test changes / Special notes to the reviewer:

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mike-hoang, schultzp2020

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