PR: Add CONTRIBUTING.md

[amilajack]

The goal with this is to make it easier for new contributors to PR to ERB. We should denote our conventions:

• PR'ing to the next branch
• Using yarn
• Use descriptive messages for PR headings and commit messages
• Denote the standards for RFCs. We can model this after Rust's RFCs
• Update CHANGELOG.md

[vikr01]

• Document [skip ci] for README changes.

This is supported by appveyor and by travis.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.