add documentation on how to use the scheduler

doc/README.Scheduler.md

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+Scheduler
+==========
+
+The scheduler in SeedDMS manages frequently run tasks. It is very similar
+to regular unix cron jobs. A task in SeedDMS is an instanciation of a task
+class which itself is defined by an extension or SeedDMS itself.
+SeedDMS has some predefined classes e.g. core::expireddocs.
+
+In order for tasks to be runnalbe, a user `cli_scheduler` must exists in
+SeedDMS.
+
+All tasks are executed by a single cronjob in the directory `utils`
+
+> */5 * * * * /home/www-data/seeddms60x/seeddms/utils/seeddms-schedulercli --mode=run
+
+Please keep in mind, that the php interpreter used for the cronjob may be
+different from the php interpreter used für the web application. Hence, two
+different php.ini files might be used. php and the php extensions may differ as
+well. This can cause some extensions to be disabled and consequently some task
+classes are not defined.
+
+`utils/seeddms-schedulercli` can also be run on the command line. If you
+do that, run it with the same system user used for the web server. On Debian
+this is www-data. Hence run it like
+
+sudo -u www-data utils/seeddms-schedulercli --mode=list