Provide developer instructions

[MarcSkovMadsen]

Developer instructions would help me and others contribute.

I would need to know

• How to install for development
• How to "build" and test
• The style guide (or automated checks used)

Example

I can see this project uses Poetry. I'm not familiar with poetry. I normally use pyproject.toml+setup.cfg+setup.py and can do pip install -e .. What would I do here to install for development?

[jmosbacher]

Solved by updating pip.