Update PATCHNOTES with warning

PATCHNOTES.md

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 # 0.20.0
 
 The changelog for this one is huge.
-Most of the changes are related to technical stuff, but the firstmost things are gameplay-related.
+Most of the changes are related to technical stuff, but the first things are gameplay-related.
 
 The reason this update took so long is mainly due to a large downtick in freetime, and making sure Datafixers were available,
 meaning you can take your old worlds up to 1.21 without anything major changing.
 
+## Remember to make a backup!
+
+While datafixing is implemented, it is still experimental. 
+You might lose item data when updating. 
+Please do report these issues on GitHub if you find them.
+
 ## Mythril Drill Rework
 
 This is an experimental rework trying to make the Mythril Drill more intuitive to use.