Update changelog

Author: oowekyala

CHANGELOG.md

@@ -6,7 +6,7 @@
 
 * **Bump required pmd-core version to 6.21.0.** The previous minimal pmd-core version was 6.12.0.
 * Add Modelica syntax highlighter, thanks to [Anatoly Trosinenko](https://github.com/atrosinenko)
-* Add GUI to export trees to XML or text format. This integrates the new experimental TreeRenderer API
+* Add GUI to export trees to XML or text format. This integrates the new experimental TreeRenderer API. (refs [#37](https://github.com/pmd/pmd-designer/pull/37))
 * Add scrollbars to code area (fixes [#22](https://github.com/pmd/pmd-designer/issues/22))
 * Internal improvements (refs [#33](https://github.com/pmd/pmd-designer/issues/33))
 * Plain text language is not loaded through the language registry anymore (see revision da47f3758)