update readme prerequisites

[olafboekholt]

I was testing a branch for Sam and noticed the readme is not completely up to date anymore.

Docker should be listed as a prerequisite.

When executing uv sync for the first time I encountered the error: pg_config executable not found.
Running "sudo apt update" and then "sudo apt install libpq-dev" (Sam's advice) installed the necessary libs.
I guess that that lib install should also be mentioned in the prerequisites/setting up section.

The rest of the readme worked fine for me.

[SilasPeters]

Epic that you didn't just say 'it be out of date' but also mentioned what was out of date!

[leuke-naam]

Disagree that any platform specific decencies should be listed, instead the README.md should mention that Postgres is needed and that one should use uv to manage Python