Update changelog - add links

Author: adangel

CHANGELOG.md

@@ -5,9 +5,9 @@
 ## 6.21.0
 
 * **Bump required pmd-core version to 6.20.0.** The previous minimal pmd-core version was 6.12.0.
-* Add Modelica syntax highlighter, thanks to @atrosinenko
-* Add scrollbars to code area (fixes #22)
-* Internal improvements (refs #33)
+* Add Modelica syntax highlighter, thanks to [Anatoly Trosinenko](https://github.com/atrosinenko)
+* 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))
 
 See https://github.com/pmd/pmd-designer/milestone/9