README: Add installation and integration docs

[dasJ]

No description provided.

[github-actions]

Nixpkgs diff

[infinisil]

Some minor improvements, but this is great!

Feel free to merge yourself when addressed :)

[infinisil]

Suggested change

	A possible configuration for `nixfmt` in `.treefmt.toml` looks like this:
	A possible configuration for `nixfmt` in `treefmt.toml` looks like this:

[infinisil]

Suggested change

	`nixfmt` was used as the basis for the official Nix formatter with a standardized formatting.
	The new formatting differs considerably from the original one.
	This is why two packages are in nixpkgs: `nixfmt-classic` with the non-standardized formatting and `nixfmt-rfc-style`.
	A recent nixfmt version is available as `pkgs.nixfmt-rfc-style` in Nixpkgs. The formatting of this version differs considerably from the original nixfmt that was used as the basis for the standardised official formatter, which is also still available as `pkgs.nixfmt-classic` for now, though unmaintained.

Just a rewording to make the distinction clearer

[infinisil]

This applies to all methods here. I think there should be an additional section referring to how the nix-shell in Nixpkgs provides the correct version for Nixpkgs. Alternatively, let's just drop this sentence, otherwise it implies the above would be the correct version. As an upstream project we shouldn't really have to document downstream uses.

Suggested change

This allows the newest changes to be used, which may not be equal to the `nixfmt` version used to format nixpkgs, so this should be done with care!

[infinisil]

Suggested change

	More information about configuration can be found in [the official README](https://github.com/numtide/treefmt-nix?tab=readme-ov-file#integration-into-nix).
	More information about configuration can be found in [the README](https://github.com/numtide/treefmt-nix?tab=readme-ov-file#integration-into-nix).

Just a minor precaution, we don't want people to think that treefmt is also an official Nix project.

[infinisil]

Suggested change

	```
	{
	outputs = { nixpkgs, self }: {
	formatter.x86_64-linux = nixpkgs.legacyPackages.x86_64-linux.nixfmt-rfc-style;
	};
	}
	```
	```nix
	{
	outputs =
	{ nixpkgs, self }:
	{
	formatter.x86_64-linux = nixpkgs.legacyPackages.x86_64-linux.nixfmt-rfc-style;
	};
	}
	```

We should adhere to our own formatting :P