Include an example ssh-key config that grabs your local key.

morph/grid/local/public-keys/users.nix.example

 # Add your public key. Example:
 # let key = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIHx7wJQNqKn8jOC4AxySRL2UxidNp7uIK9ad3pMb1ifF flo@fs-la";
+# You can use the following to get key from the local machine.
+# let key = builtins.readFile ~/.ssh/id_ed25519.pub;
 let key = undefined;
 in { "root" = key; "vagrant" = key; }