Added automatic actions based on a daily event

doc/cronjob.markdown

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+Background Job Scheduling
+=========================
+
+To work properly, Kanboard requires that a background job run on a daily basis.
+Usually on Unix platforms, this process is done by `cron`.
+
+This background job is necessary for these features:
+
+- Reports and analytics (calculate daily stats of each projects)
+- Send overdue task notifications
+- Execute automatic actions connected to the event "Daily background job for tasks"
+
+Configuration on Unix and Linux platforms
+-----------------------------------------
+
+There are multiple ways to define a cronjob on Unix/Linux operating systems, this example is for Ubuntu 14.04.
+The procedure is similar to other systems.
+
+Edit the crontab of your web server user:
+
+```bash
+sudo crontab -u www-data -e
+```
+
+Example to execute the daily cronjob at 8am:
+
+```bash
+0 8 * * * cd /path/to/kanboard && ./kanboard cronjob >/dev/null 2>&1
+```
+
+Note: the cronjob process must have write access to the database in case you are using Sqlite.
+Usually, running the cronjob under the web server user is enough.