Add exfat warning for USB thumb drives (#1444)

By default, exfat drives are mounted owned by root, with 755 perms
on the mountpoint.  Rather than going into setting 'uid' and 'gid'
params, which rely on the uid and gid of the kvm user never changing,
I thought it would be simpler to just give users the hint to set the
default umask to 0000 (aka, 777) for all files. This allows the
thumb drive to be visible as read-write.

Signed-off-by: Rob Thomas <xrobau@gmail.com>

docs/msd.md

@@ -220,6 +220,24 @@ To refresh the list of available isos on the share it is currently necessary to
 
     3. Perform `reboot` to apply all changes.
 
+-----
+### 'exfat' Filesystem warning
+
+Using the existing USB ports you can reduce writes to the internal SSD card by storing
+images on a USB thumb drive.  This is mounted as would NFS or Samba, above. As recent
+drives are starting to come formatted with `exfat` instead of `fat` or `NTFS`, the
+linux kernel will default to 'root' ownership of the mountpoint.  This means the
+volume will be visible in the 'Media' menu, but will be marked as `[read-only]`, without
+any obvious reason.
+
+The correct `/etc/fstab` entry for a USB drive that presents as `/dev/sda1` with an
+exfat filesystem is:
+
+        ```fstab
+        /dev/sda1   /var/lib/kvmd/msd/usb   exfat  auto,nofail,rw,umask=0000  0 0
+        ```
+This says to mount it automatically, do not fail if it's missing, mount it read/write by
+default, and allow all users and groups access to it.
 
 -----
 ## Multiple drives