Add better contributing instructions

[MrPowers]

Tell users how to get started with mack on their local machine.

• install poetry
• install the dependencies with poetry
• run the test suite
• anything else?

This is the file that needs to be updated.

[Triamus]

Not sure if that is feasible but a ready-to-use docker image would be great. As alternative for what you already listed above.

[MrPowers]

@Triamus - a ready-to-use Docker image is a great idea. Can you please create a separate issue so we can track that request and get it done? Thanks!

[Triamus]

@MrPowers done in #72 .