Fix README to make usage of poetry clear

[clemens-tolboom]

The idea is we deliver a correct installation guide ... maybe this is a good moment to replace the content with this first?

[clemens-tolboom]

Ie https://discordapp.com/channels/998688164514365593/998688167140016294/999250476463816704 should be documented too

Hella easy for poetry, poetry install, for the pre commit hooks I usually do pre-commit install --install-hooks