pdate demo program

needed a mkdir

doc/design/compute_special_remote_interface.mdwn

@@ -103,5 +103,6 @@ An example `git-annex-compute-foo` shell script follows:
     echo "OUTPUT $3"
     echo REPRODUCIBLE
     if [ -n "$input" ]; then
+        mkdir -p "$(dirname "$3")"
         frobnicate --passes="$ANNEX_COMPUTE_passes" <"$input" >"$3"
     fi