Add comparison URL for the v0.12.4 release

Author: arqunis

CHANGELOG.md

@@ -5517,6 +5517,7 @@ Initial commit.
 
 <!-- COMPARISONS -->
 
+[0.12.4]: https://github.com/serenity-rs/serenity/compare/v0.12.3...v0.12.4
 [0.12.3]: https://github.com/serenity-rs/serenity/compare/v0.12.2...v0.12.3
 [0.12.2]: https://github.com/serenity-rs/serenity/compare/v0.12.1...v0.12.2
 [0.12.1]: https://github.com/serenity-rs/serenity/compare/v0.12.0...v0.12.1