Update RELEASE_NOTES.md for 0.10.2 release

Author: Arkatufus

RELEASE_NOTES.md

@@ -1,2 +1,6 @@
+### 0.10.2 June 30 2021 ####
+* [Update Akka version to 1.4.21](https://github.com/akkadotnet/akka.net/releases/tag/1.4.21)
+* [Add exception rethrow to help with debugging](https://github.com/akkadotnet/Hyperion/pull/229)
+
 ### 0.10.1 April 20 2021 ####
 * [Fix SerializerOptions constructor backward compatibility issue with Akka.NET](https://github.com/akkadotnet/Hyperion/pull/214)