Update CHANGES.txt to include the description of #7210 (#7212)

diff --git a/CHANGES.txt b/CHANGES.txt
index 3da8143..67c5a3d 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -2,6 +2,7 @@
   
   C#
   * Fix latest ArgumentException for C# extensions (#7188)
+  * Enforce recursion depth checking for unknown fields (#7210)
   
   Ruby
   * Fix wrappers with a zero value (#7195)