Add UPGRADE.md as internal upgrade documentation

CHANGES.md

@@ -6,6 +6,7 @@ Changes
 
 ### Unreleased
 
+* 2022-11-26 - Add UPGRADE.md as internal upgrade documentation
 * 2022-11-26 - Update maintainers and copyrights in README.md.
 * 2022-11-26 - Change in Moodle release support:
                For the time being, this plugin is maintained for the most recent LTS release of Moodle as well as the most recent major release of Moodle.

UPGRADE.md

@@ -0,0 +1,35 @@
+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 the most important of the plugin's user stories.
+
+
+Manual tests
+------------
+
+* As the plugin deals with HTML changes in the rich-text-editor, manual tests should be carried out to see if the plugin really keeps working as expected on new Moodle major versions.
+
+
+Visual checks
+-------------
+
+* As the plugin's purpose is to apply particular styles to text in the rich-text-editor, manual tests should be carried out to see if the plugin really looks right on new Moodle major versions.