Clarify support for fish shell

[jab]

I recently installed nix-darwin for the first time. The installer mentioned that it was modifying config for bash and zsh to interoperate with nix, but it didn't say anything about fish, so I'm not sure to what extent fish is supported, and what steps I need to take to use nix-darwin-provided fish as my default shell.

I tried adding pkgs.fish to environment.systemPackages and setting programs.fish.enable = true in my darwin-configuration.nix, then running darwin-rebuild switch && echo /run/current-system/sw/bin/fish > /etc/shells && chsh -s /run/current-system/sw/bin/fish, and finally opening a new terminal window, and immediately hit #122, so fish was unusable out-of-the-box. Once I added this workaround I got it working.

1. Since the installer mentions bash and zsh, would it be worth mentioning fish as well (even if only to parenthetically say e.g. "If you want to use fish, see #122" (or whatever other link makes sense) until there's better support for fish, since there are quite a few fish users out there who might have a similar experience?
2. If the steps I took (above) for setting up nix-darwin-provided fish as my shell were correct, want me to contribute them somewhere that would be more discoverable to fish users than here? If they weren't correct, if I find out what the correct steps are I could write those down somewhere better instead.

Thanks for maintaining nix-darwin!