add short paragraph linking to hook script

Author: felixschurk

content/docs/using_dates.md

@@ -24,6 +24,11 @@ Using our example, we can set the `due` date to be Alice's birthday
                due:2016-11-08
 
 Now your task has an associated `due` date, to help you determine when you need to work on it.
+
+The default `due` time is at 0:00 on the date specified.
+A hook for configuring this time is [https://github.com/tbabej/task.default-date-time](https://github.com/tbabej/task.default-date-time).
+See [hooks_guide](docs/hooks_guide.md) for details of hooks.
+
 But what effect does this have on Taskwarrior? How can it be used to best advantage?
 
 We call the `due` date of a task 'metadata'.