zonefs: Add documentation

Add the new file Documentation/filesystems/zonefs.txt to document
zonefs principles and user-space tool usage.
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

MAINTAINERS

@@ -18311,6 +18311,7 @@ L:	linux-fsdevel@vger.kernel.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/dlemoal/zonefs.git
 S:	Maintained
 F:	fs/zonefs/
+F:	Documentation/filesystems/zonefs.txt
 
 ZPOOL COMPRESSED PAGE STORAGE API
 M:	Dan Streetman <ddstreet@ieee.org>