cal: basic README added

cal/README.md

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+Calendar Export
+===============
+
+This addon makes it possible to export the events posted by your users being
+exported to other formats, so they can be imported into other calendar
+applications.
+
+Exporting a calendar is an _opt-in_ feature which has to be activated by each
+of the users in the _addon settings_. As the admin you can only provide the
+service but should not force it upon your users.
+
+The calendars are exported at
+
+	http://example.com/cal/username/format
+
+currently the only supported format is __ical__.
+
+Author
+------
+
+This addon is developed by [Tobias Diekershoff](https://f.diekershoff.de/profile/tobias).
+
+License
+-------
+
+This addon is licensed under the [MIT](http://opensource.org/licenses/MIT)
+license, see also the LICENSE file in the addon directory.