Add doc about recurring tasks

docs/recurring-tasks.markdown

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+Recurring tasks
+===============
+
+- Recurring tasks are duplicated to the first column of the board when the selected events occurs
+- The due date can be recalculated automatically
+- Each task records the task id of the parent task that created it and the child task created
+
+Configuration
+-------------
+
+Go to the task view page or use the dropdown menu on the board, then select **Edit recurrence**.
+
+![Recurring task](http://kanboard.net/screenshots/documentation/recurring-tasks.png)
+
+There are 3 triggers that currently create a new recurring task:
+
+- Moving a task from the first column
+- Moving a task to the last column
+- Closing the task
+
+Due dates, if set on the current task, can be recalculated by a given factor of days, months or years.
+The base date for the calculation of the new due date can be either the existing due date, or the action date.