Update CONTRIBUTING.md like WordPress-Android repo

[hasanuzzamanbe]

Expected behavior

Note: If you are part of the org and have the permissions on the repo, don't forget to assign yourself to the PR, and add the appropriate GitHub label and Milestone for the PR. If you are not part of the org, you cannot directly merge changes to the main repo. In that case, you'll need to fork the repo before you can submit a PR.

Actual behavior

Note: If you are part of the org and have the permissions on the repo, don't forget to assign yourself to the PR, and add the appropriate GitHub label and Milestone for the PR

Steps to reproduce the behavior

Mentioned in issue wordpress-mobile/WordPress-Android#15290
on WordPress-Android repo.

Tested on [device], iOS [version], WPiOS [version]