Add contributing.md file to repository

# Background

We need a short guide on how to contribute to this project. The `CONTRIBUTING.md` file usually holds all the relevant information contributors need to know before starting their contributions to this project.

# Requirements

Add a `CONTRIBUTING.md` file that contains:

- how to set up the development environment,
- the open source licenses to which contributors are agreeing,

# Resources

Here is an example from the Gradle project https://github.com/gradle/gradle/blob/master/CONTRIBUTING.md