Add performance improvement section to UPGRADING

[ci skip]

Author: nikic

UPGRADING

@@ -13,6 +13,7 @@ PHP 7.4 UPGRADE NOTES
 11. Changes to INI File Handling
 12. Windows Support
 13. Other Changes
+14. Performance Improvements
 
 
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@@ -224,3 +225,13 @@ PHP 7.4 UPGRADE NOTES
 ========================================
 13. Other Changes
 ========================================
+
+========================================
+14. Performance Improvements
+========================================
+
+- Core:
+  . A specialized VM opcode for the array_key_exists() function has been added,
+    which improves performance of this function if it can be statically
+    resolved. In namespaced code, this may require writing \array_key_exists()
+    or explicitly importing the function.