add trace to CHANGELOG.md

Author: thatfiredev

firebase-perf/CHANGELOG.md

@@ -1,5 +1,15 @@
 # Unreleased
 
+* 
+
+## Kotlin
+The Kotlin extensions library transitively includes the updated
+`firebase-performance` library. The Kotlin extensions library has the following
+additional updates:
+
+* [feature] Added a [`trace(String, Trace.() -> T)`](https://firebase.google.com/docs/reference/kotlin/com/google/firebase/perf/ktx/package-summary#trace(kotlin.String,kotlin.Function1))
+ extension function to create a custom trace with the given name.
+
 # 20.2.0
 * [unchanged] Updated to accommodate the release of the updated
   [perfmon] Kotlin extensions library.