docs: Adds documentation for historical events.

README.md

@@ -2,7 +2,8 @@
 > Emits [xAPI](https://github.com/adlnet/xAPI-Spec/blob/master/xAPI.md) statements using the [Moodle](https://moodle.org/) Logstore.
 
 - Install the plugin using [our zip installation guide](/docs/install-with-zip.md).
-- Ask questions with the [Gitter chat room](https://gitter.im/LearningLocker/learninglocker)
+- Process events before the plugin was installed using [our historical events guide](/docs/historical-events.md).
+- Ask questions with the [Gitter chat room](https://gitter.im/LearningLocker/learninglocker).
 - Report bugs and suggest features with the [Github issue tracker](https://github.com/xAPI-vle/moodle-logstore_xapi/issues).
 - View the supported events in [our `get_event_function_map` function](/src/transformer/get_event_function_map.php).
 - Change existing statements for the supported events using [our change statements guide](/docs/change-statements.md).