pkgs/README.md: classify and add rules to the naming convention

[Aleksanaa]

Description of changes

This is noticed in NixOS/nixpkgs-vet#107, and we should explicitly tell contributors how to deal with variables starting with a digit.

Rendered

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[fricklerhandwerk]

I'm in favor of the semantic change. Thanks for taking the time @Aleksanaa!

Left a bunch of purely formal suggestions.

[Aleksanaa]

@fricklerhandwerk Would you give a final review? It looks good from my side now.

I'd also like to refactor more of this README to make the structure clear, but I don't want to expand the scope of this PR too much

[fricklerhandwerk]

If you have more clarifications for this document, I'm glad to review those, too. This is a dense text and it doesn't exactly make contributions easier if you want to follow all the rules.

[Aleksanaa]

This is what I had in mind. IMO it's logically better, but conventions goes to bottom and I'm afraid less people would carefully read it (there's side pane on GitHub tho). I didn't modify most of texts here, just rearranging it, so the wording problem should be omitted. Only this is newly added:

The pkg-name shall be the top-level attribute name used for the package, such as python3Packages.setuptools-rust, nixVersions.nix_2_24 and alpaca. This is to trigger ofborg build on the package added or modified.

I have to go to sleep now. It would be helpful if you can express a brief opinion on this. Thanks!

[fricklerhandwerk]

Oh, slight misunderstanding... let's do further changes in a follow up PR. I was ready to merge this.

[Aleksanaa]

@fricklerhandwerk dropped