README: better instructions on how to run zfs-fuse on Fedora

Make will not fail if zfs-fuse is missing (it will just print some
strange messages), but the resulting image will fail. It would be
nice if the makefile checked that our use of zfs-fuse is actually
working, but let's start with instructions on the README on how to
enable it in a way that survives reboot :-)

README

@@ -36,7 +36,8 @@ To build OSv
    ----------------------------------------
 
    On Fedora:
-     sudo service zfs-fuse start
+     sudo systemctl start zfs-fuse.service
+     sudo systemctl enable zfs-fuse.service	# to have it start on reboot
 
    On Debian the daemon should be started automatically.