I added this to my `~/.spacemacs` file to turn on relative line numbers when in Python mode:
I had to move this outside of the user `(defun dotspacemacs/user-config () ... )` config section. It doesn't work if you put it in there.
This example uses python. It works with other modes as well. I confirmed the name of the hook finding the name of the variable via `C-h v` then typing `hook` and scanning the results to find what I'm looking for.