Add FIAM release notes for M71 (#5580)

* Add release notes for M71

* Fix version number

Author: christibbs

FirebaseInAppMessaging/CHANGELOG.md

@@ -1,3 +1,6 @@
+# 2020-05-19 -- v0.20.1
+- [fixed] Fixed an issue where clicks were counted for messages with no action URL (#5564).
+
 # 2020-04-21 -- v0.19.3
 - [fixed] Fixed an issue where GoogleUtilities wasn't explicitly listed as a dependency (#5282).