Developer contribution guide

[ytsarev]

We need to create contribution guide which would capture development topics like

• Useful development make targets
• unit testing approach
• end-to-end testing approach

This chunk of documentation will be useful both for us and external contributors.

[somaritane]

@ytsarev Does CONTRIBUTING.md reflect the requirements from this issue already? If yes, we can close the issue.

[somaritane]

Closing as we have https://github.com/k8gb-io/k8gb/blob/master/CONTRIBUTING.md in place for quite some time