Instructions to test: add sample shell.nix

[lucasew]

If we just do that nix-store -r it may be good enough because of testing binaries and so.

But, for example, in the case of [1] I had to add the package to a shell, so Blender's python would detect it, and then I could proceed to test it.

I've pasted the shell.nix I've used, and I think it would be very useful to provide a snippet like this to help, mainly, less experienced maintainers, or experienced maintainers that want to spend less time in their work.

[1] NixOS/nixpkgs#219545