Bump version, add build docs for pypi release

[samuelallan72]

I still maintain the sncli pypi release ( https://pypi.org/project/sncli/ ), as I've been unable to hand it off yet.

So I cut a new minor release from master and pushed it to pypi: 0.4.3. Seems safe as there have been no new changes since mid 2023, and things sound fairly stable.

Diff: https://github.com/insanum/sncli/compare/0.4.2...samuelengineer:sncli:0.4.3-pypi

Also added build and release instructions to the readme for future reference.

Happy to hand over pypi maintainership to maintainers here. :)