Add 'Migration'

Migration.md

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+## Migration
+
+This plugin should be compatible with configurations from the Stargate plugin all the way back. The nethergate.gate 
+file, the endgate.gate file and the watergate.gate file will be overwritten if they exist. Take a backup of the files 
+and overwrite the files after the first startup if you want to keep your custom gates.
+
+If you have legacy gate files using the old numeric material ids, you need to change them to the new format manually. 
+Use F3 + H to see material ids. Use them exactly as written after "minecraft:".
+The configuration will be updated to a more easily readable format, but the old configuration will be saved in case you 
+want to change back right away.
+
+Permissions have had a few changes, so you should check the permissions section for any differences since you set up 
+permissions.