Document editorconfig and nixfmt, remove manual rules

[infinisil]

Description of changes

This documents editorconfig and nixfmt in CONTRIBUTING.md. Ping @NixOS/nix-formatting @eclairevoyant

---

Add a 👍 reaction to pull requests you find important.

[Sigmanificient]

CI also checks for files that already exists

Suggested change

	- New files must be formatted by entering the `nix-shell` from the repository root and running `nixfmt`.
	- New and edited files must be formatted by entering the `nix-shell` from the repository root and running `nixfmt`.

[eclairevoyant]

The check is for new and already-formatted files, not all edited files.

But it's probably more confusing to say "already-formatted" as you have to explain how to check that in turn... and that ratchet will eventually tighten anyway

[Sigmanificient]

yeah, "Loop through all Nix files touched by the PR",
maybe New and Edited files?

[eclairevoyant]

But again, it's not all edited files, nor do we necessarily want people formatting, say,all-packages.nix or other unformatted files with a high chance of merge conflicts

I think the sentence is fine as-is; if there's an issue with a file, CI will flag it and it's the same process to rectify it.

[Sigmanificient]

Any suggestion then?

[infinisil]

As @eclairevoyant also said, it's not all edited ones, see #326407 for more context :)

I'd say it's okay to just say "New files" here, for now leaving it undefined how existing files should be formatted. Once we enforce it for all files (probably in like 2 months or so), we can change this to "All files".

Edit: Oh we're fast with replies, didn't see your replies in advance!

[Sigmanificient]

Okay, just wanted to be sure that this was mentioned, just in case

[JohnRTitor]

Just so we are clear, nixfmt in the nix-shell is from the nixfmt-rfc-style package, right?

[eclairevoyant]

Thank you for documenting the change so quickly!

[infinisil]

Thanks for the quick reviews, I see no reason to wait with merging this, let's go! 🚀