Motivation for this change

So I've been reviewing a few PRs recently and I found that it's actually fairly common for these attributes to be left out (for PRs adding new packages). I know that CONTRIBUTING.md already has a link to the nixpkgs manual which mentions these attributes, but I felt that maybe these two lines could be added to CONTRIBUTING.md directly as well, seeing as though people seem to omit them from time to time?

The manual does say that a maintainer must be set, but I don't think the manual explicitly mentions that a license attribute must be set. However, even if the manual doesn't say a license must be set, I believe it's good form to require it for all packages (hence the line about meta.license), thoughts? For example, even if a package does not have a license, it can just be assumed to be nonfree (because I believe that's legally the case in basically all parts of the world?), and therefore have a license attribute stdenv.lib.licenses.unfree.

Just to clarify, obviously meta attributes are optional, but I'm talking about when a package is to be added to this nixpkgs repository.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.