Added notice to UPGRADING

Author: pmmaga

UPGRADING

@@ -24,6 +24,9 @@ PHP 8.0 UPGRADE NOTES
   . Removed track_errors ini directive. This means that $php_errormsg is no
     longer available. The error_get_last() function may be used instead.
   . Removed create_function(). Anonymous functions may be used instead.
+  . Any array that has a number n as its first numeric key will use n+1 for
+    its next implicit key. Even if n is negative.
+    RFC: https://wiki.php.net/rfc/negative_array_index
 
 - GD:
   . The deprecated function image2wbmp() has been removed.