Installation of just in README.md

[enderversing]

The installation instructions in the README.md should explain that just needs to be installed, and how to install it!

(I find the just installation instructions to not be straightforward, so I'm not sure how to make this change myself.)

[catilac]

Hi @enderversing thanks so much for creating this issue!
You can install just with: cargo install just on the command line if you have your rust toolchain installed already. let me know if you need help with any of that.

If that works, I can assign this issue to you if you change your mind!

Please report back :)