Update release notes wrt #687

Added reference to the amount of improvement.

Author: k163377

release-notes/VERSION-2.x

@@ -19,6 +19,8 @@ Co-maintainers:
 2.16.0 (not yet released)
 
 #687: Optimize and Refactor KotlinValueInstantiator.createFromObjectWith.
+ This improves deserialization throughput about 1.3 ~ 1.5 times faster.
+ https://github.com/FasterXML/jackson-module-kotlin/pull/687#issuecomment-1637365799
 #685: Streamline default value management for KotlinFeatures.
  This improves the initialization cost of kotlin-module a little.
 #684: Kotlin 1.5 has been deprecated and the minimum supported Kotlin version will be updated to 1.6.