Make instructions about tree & pre-commit deps explicit

[ErikBjare]

@jrmi tried it and said on Discord:

Congrats! Looks great 🚀

FYI (and if anyone experiences the same issues), I had to execute these two commands to make it work with my linux:
$ pipx install pre-commit # For the ./fork.sh command
$ sudo apt install tree # For the ./run.sh