payjoin-cli: Update README nigiri install instructions

[thebrandonlucas]

It appears it is no longer possible to download nigiri via cargo install. This replaces it with the recommended bash script from Github.

Also, my markdown linter made a couple tiny edits (removed the period from the title), and I didn't undo them since I trust the linter knows opinionatedness, but can undo anything if we don't like it.

[coveralls]

Pull Request Test Coverage Report for Build 14799132323

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 82.057%

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Totals
Change from base Build 14760789160:	0.0%
Covered Lines:	5433
Relevant Lines:	6621

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💛 - Coveralls

[DanGould]

Despite our use of docker in tests, I am somewhat of a nigiri hater myself. Hope you didn't have to sink too much time on this versus what it'd take to make a flake work.

ACK 21648af

[thebrandonlucas]

Despite our use of docker in tests, I am somewhat of a nigiri hater myself. Hope you didn't have to sink too much time on this versus what it'd take to make a flake work.

ACK 21648af

@DanGould Little time thankfully, but if that's already where your head is at then yes I'd be happy to explore setting up a nix flake or a docker build for this. Since it seems we make use both nix and docker, I'm not sure which would be more beneficial and aligned with the current state of the project. Biased toward nix as that would allow the binary itself to be installed easily and lighter weight than using docker, but docker is more widely adopted and portable.

Could also pursue both, let me know what you think is best.

Regardless, too many steps currently with nigiri.