When I first tried to build FreeNAS, I encountered multiple
random build errors during the building of ports when
the build tried to unmount certain nullfs file systems.
It turns out that the devel/gamin port installed on my system
(which gets pulled in via the devel/glib20 port) tries
to poll mounted file systems to monitor file system activity, and sometimes
gets stuck, preventing the file system from unmounting.

Add a note to the README explaining how to disable gamin,
to avoid this problem when building FreeNAS.