Add basic readme to setup a nix flake for standalone payjoin-mailroom

[benalleng]

This is a minimal readme for recreating the nix flake to run a payjoin-mailroom instance in standalone nix systems.

The settings added here are not intended to actually produce a functional setup, those will come as a followup, this PR is only intended to show the format for how a flake should be organized.

Pull Request Checklist

Please confirm the following before requesting review:

• I have disclosed my use of
  AI
  in the body of this PR.
• I have read CONTRIBUTING.md and rebased my branch to produce hygienic commits.

[coveralls]

Pull Request Test Coverage Report for Build 22767683746

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 82.497%

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Change from base Build 22741440695:	0.0%
Covered Lines:	10638
Relevant Lines:	12895

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💛 - Coveralls

[spacebear21]

cACK, a few comments on specifics below

[spacebear21]

This should probably point to a tag (and so should docker-compose.toml for that matter).

[DanGould]

I'm pretty bearish on READMEs as documentation without deliberate plan to prevent bitrot from the getgo. Less in it, less to rot.

Concrete mitigations

• Is CI-test of example flake cost-prohibitive or possible (I comment a suggestion)
• Totally agree with @spacebear21's suggestion to relative link example flake If the file moves, the link breaks in GitHub.
• Omit the version pin from the example or use a placeholder. The nixos-25.11 and payjoin-mailroom-0.1.0 in the example flake will age quickly. Better to either use a comment like # pin to your preferred release or the nixos-unstable channel, with a note directing users to the releases page for stable pins.
• Keep the config reference minimal and point to the canonical file. The current approach of just saying "see config.example.toml" is it.
• "last tested" datestamp or version badge if CI isn't feasible is probably fine

[DanGould]

This is out of date immediately. If we used

Suggested change

	rust-payjoin = {
	url = "github:payjoin/rust-payjoin/tree/payjoin-mailroom-0.1.0";
	};
	rust-payjoin.url = "github:payjoin/rust-payjoin";

and the local path in CI

nix flake check ./nix/modules/example \
  --override-input rust-payjoin path:$(pwd) \
  --no-write-lock-file

We would make sure this example actually keeps working. Could use big ### pin to your preferred release or the nixos-unstable channel. see <release link> for versions in the example if really necessary.

[benalleng]

Happy to make this as minimal as possible to avoid this ever getting stale, my intention with the flake.example.nix is only to demonstrate how a basic flake should be setup (inputs, outputs, etc) and to bring attention to the fact that we have the module to support this.

[DanGould]

Suggested change

	ruts-payjoin,
	rust-payjoin,

[DanGould]

Just track master and ref latest with a comment?

Suggested change

	rust-payjoin.url = "github:payjoin/rust-payjoin/tree/<payjoin-mailroom-latest>";
	rust-payjoin.url = "github:payjoin/rust-payjoin";

Robot says /tree is web UI path only and github: shorthand for a tag here would be

rust-payjoin.url = "github:payjoin/rust-payjoin/<payjoin-mailroom-0.1.0>";

I'm inclined to believe