Add upgrade note

alcaeus · alcaeus · commit 9864dbcc7bcb · 2024-09-20T10:10:28.000+02:00
diff --git a/UPGRADE-2.0.md b/UPGRADE-2.0.md
@@ -2,6 +2,7 @@ UPGRADE FROM 1.x to 2.0
 ========================
 
  * Classes in the namespace `MongoDB\Operation\` are `final`.
+ * All methods in interfaces and classes now define a return type.
 
 GridFS
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