Add missing link to PR 78

Author: lasselindqvist

CHANGELOG.md

@@ -7,10 +7,11 @@
 
 ## 2.1.0 (released 2019-06-08)
 - Update minimum Java version to 1.6.
-- Switch to proguard 6.0.3 as the default version.
+- Switch to proguard 6.0.3 as the default version. ([78][])
 - Add support for incremental obfuscation ([73][])
 
 [73]: https://github.com/wvengen/proguard-maven-plugin/pull/73
+[78]: https://github.com/wvengen/proguard-maven-plugin/pull/78
 
 ## Older versions
 - Check the commit history for details.