Add Kubeflow Contributor Guide to CONTRIBUTING.md

[davidgs]

This document provides guidelines for contributing to the Kubeflow project, including links to the contributor's guide and details on the contribution process.

Which issue is resolved by this Pull Request:

#1063

Description of your changes:

Adds a CONTRIBUTING.md file as required.

Checklist:

If PR related to Optional-Test-Infra,

• Changes need to be generated to aws/GitOps folder:
  1. cd aws
  2. make optional-generate
  3. make optional-test

[google-oss-prow]

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