[ci skip] Update UPGRADING

cmb69 · cmb69 · commit a9ab5d473b9e · 2018-04-04T14:11:37.000+02:00
5c5bd30 removed support for `--with-libmbfl`.
diff --git a/UPGRADING b/UPGRADING
@@ -169,6 +169,9 @@ JSON:
  FTP:
   . Set default transfer mode to binary
 
+ MBString:
+  . The configuration option --with-libmbfl is no longer available.
+
  ODBC: 
   . Support for ODBCRouter has been removed.
   . Support for Birdstep has been removed.
