Add UPGRADE.md to the repository.

UPGRADE.md

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+Upgrading this plugin
+=====================
+
+This is an internal documentation for plugin developers with some notes what has to be considered when updating this plugin to a new Moodle major version.
+
+General
+-------
+
+* Generally, this is a quite simple plugin with just one purpose.
+* It does not rely on any fluctuating library functions and should remain quite stable between Moodle major versions. 
+* Thus, the upgrading effort is low.
+
+
+Upstream changes
+----------------
+
+* This plugin does not inherit or copy anything from upstream sources. 
+
+
+Automated tests
+---------------
+
+* The plugin has a good coverage with Behat tests which test all of the plugin's user stories.
+
+
+Manual tests
+------------
+
+* There aren't any manual tests needed to upgrade this plugin.