Add contributor's guide

[ttarczynski]

We need a guide that will help new contributors when working on their first pull request. Such guide should:

• describe the contributing process (e.g. issues, milestones, submitting a pull request, etc.)
• list all conventions and requirements for the examples
  • see: README.md > conventions
• explain the principles of this project
  • see: README.md > principles
• explain how the CI checks work and explain how to run them locally
  • see: README.md > github-actions-workflow

Some information are already in README.md, but we need to make it easier for first-time contributors.

[teszes]

Should this be separated into a "CONTRIBUTING.md"?

[ttarczynski]

Yes, I think CONTRIBUTING.md would be the way to go (and link it in README.md).