INSTALL.md: Remind users to check `which xmonad`
INSTALL.md: Remind users to check `which xmonad`

We have had this situation happen a few times now: users update
xmonad (say, to 0.17.0) but forget that they still have an older version
installed via the distributions repositories.  Features that depend on
the "bootstrap" xmonad executable to be updated (like the improved XDG
support) then fail badly.

Thus, remind users to check whether the right executable is present.

xmonad/xmonad