btrfs-progs: docs: add example on deleting a subvolume

Perhaps a rather trivial example but I find it helpful to see usage
examples. Maybe it will help others.

Pull-request: #244
Author: Alexandru Ungureanu <khakcarot@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/btrfs-subvolume.asciidoc

@@ -233,6 +233,20 @@ are completed, but do not wait for subvolumes deleted in the meantime.
 -s <N>::::
 sleep N seconds between checks (default: 1)
 
+EXAMPLES
+--------
+
+.Deleting a subvolume
+====
+If we want to delete a subvolume called `foo` from a btrfs volume mounted at
+`/mnt/bar` we could run the following:
+
+[source,bash]
+----
+btrfs subvolume delete /mnt/bar/foo
+----
+====
+
 EXIT STATUS
 -----------
 *btrfs subvolume* returns a zero exit status if it succeeds. A non-zero value is