Add "skhd --help" flag?

When trying to solve [this issue](https://github.com/koekeishiya/skhd/issues/285), I tried to list all flags available in skhd but none of these worked:

- `man skhd`
- `skhd --help`
- `skhd -h`

Would it make sense to add a help page, or if it already exists, alias it to `--help`?

Start by locating skhd's command-line argument entry point and checking whether a man page or existing help text is present. Compare the available flags with the documented behavior, then verify that the requested help invocation and any proposed alias display the available options.

Written by the indexing model from the issue text.