Document --rootdir mkfs switch

index 542e6cf..25e817b 100644
@@ -12,6 +12,7 @@ mkfs.btrfs \- create an btrfs filesystem
 [ \fB\-M\fP\fI mixed data+metadata\fP ]
 [ \fB\-n\fP\fI nodesize\fP ]
 [ \fB\-s\fP\fI sectorsize\fP ]
+[ \fB\-r\fP\fI rootdir\fP ]
 [ \fB\-h\fP ]
 [ \fB\-V\fP ]
 \fI device\fP [ \fIdevice ...\fP ]

@@ -59,6 +60,9 @@ Specify the nodesize. By default the value is set to the pagesize.
 \fB\-s\fR, \fB\-\-sectorsize \fIsize\fR
 Specify the sectorsize, the minimum block allocation.
 .TP
+\fB\-r\fR, \fB\-\-rootdir \fIrootdir\fR
+Specify a directory to copy into the newly created fs.
+.TP
 \fB\-V\fR, \fB\-\-version\fR
 Print the \fBmkfs.btrfs\fP version and exit.
 .SH AVAILABILITY