Add note about Emacs package just-mode

[leon-barrett]

I added an Emacs package for editing justfiles, just-mode, and I thought it might be worth including in the README file.

[casey]

That's so cool! Thanks so much for writing it!

This is very cool! So if you edit a recipe in a justfile that uses differently configured tabs, it will insert the pre-existing indent?

[leon-barrett]

Thanks. It's not quite as cool as you make it sound--it's more like, while you're in a rule, it'll keep matching the indent of the previous line. When you start a new rule, it'll just use the default 4 spaces.

I believe that's helpful for writing things like Python scripts where you want varying levels of indentation within a rule. It's not super graceful, but it does something at least. (That's why there's a TODO to use nested modes for better scripting, but it's still a TODO because honestly just getting the syntax highlighting nice was hard enough for now.)

[casey]

Still very nice. "It does something at least" is often the best that one can hope for!