docs: add mount --namespace to TODO

Signed-off-by: Karel Zak <kzak@redhat.com>

Documentation/TODO

@@ -45,6 +45,10 @@ bash completion
 libmount (mount/umount)
 -----------------------
 
+ - allow to execute mount(2) in another namespace, something like:
+	mount --namespace=/proc/$n/ns/mnt  /dev/sda2 /bar
+   see https://bugzilla.redhat.com/show_bug.cgi?id=1199554 for more details.
+
  - add options to control fstab/mtab mount options usage, something like:
 
    --options-mode={ignore,append,prepend,replace}      MNT_OMODE_{IGNORE, ...}