Custom keybinding

Put this in the config file:

bindcode $mod+<keycode> exec <command>

The keycode can be determined with xev. Start xev and press the key you want to know the keycode of. The output for the key p should be similar to the following:

KeyRelease event, serial 32, synthetic NO, window 0x1e00001,
  root 0x264, subw 0x0, time 3882658563, (471,53), root:(2395,701),
  state 0x10, keycode 33 (keysym 0x70, p), same_screen YES,
  XLookupString gives 1 bytes: (70) "p"
  XFilterEvent returns: False

A valid code line looks then like this:

bindcode $mod+33 exec gimp