Updated changelog.

CHANGES.txt

diff --git a/CHANGES.txt b/CHANGES.txt
index 9037867..5bad89e 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -5,6 +5,7 @@
   * Fix bug where `Descriptor::DebugString()` printed proto3 synthetic oneofs.
   * Provide stable versions of `SortAndUnique()`.
   * Make sure to cache proto3 optional message fields when they are cleared.
+  * Expose UnsafeArena methods to Reflection.
 
   Kotlin
   * Restrict extension setter and getter operators to non-nullable T.