Add examples/slightly more verbose docs for hooks

[TLATER]

I've finally switched over to genkernel-next, and I wanted to figure out if I could use it to automatically sign my kernel and move the latest to EFI/BOOT/bootx64.efi post-compilation.

Looking at #62, this seems very possible, but I can't figure out how to actually find my kernel and such - considering the implementation it looks like some variables will be carried over that could help with that, but it's a little hard to track down what's actually going on.

The documentation is unfortunately not very helpful beyond pointing out that it is possible to create hooks.