Add a contributing guideline

[halsk]

Is your feature request related to a problem? Please describe.

It is important to show how contributors can contribute to this project.

Describe the solution you'd like

Add docs/CONTRIBUTING.md and write how to contribute, then add a link from README.

Additional context

Example from other repositories.

• The Atom editor contribution guidelines.
• The Ruby on Rails contribution guidelines.
• The Open Government contribution guidelines.