croutonxinitrc-wrapper: Maximize Xephyr on startup

Recent versions of Xephyr (>=1.15) do not start in full screen:
tell croutonwmtools to maximize it.

Fixes #744.
croutonxinitrc-wrapper: Update comment.

Indicate which versions of Xephyr are affected, so that the hack
can be removed later on.
Merge pull request #750 from drinkcat/xephyr-fullscreen

croutonxinitrc-wrapper: Maximize Xephyr on startup