Update RELEASE_NOTES.md for 1.5.4 release (#210)

Author: Arkatufus

RELEASE_NOTES.md

@@ -1,3 +1,6 @@
+#### 1.5.4 June 16 2021 ####
+* Upgraded to [Akka.NET v1.4.21](https://github.com/akkadotnet/akka.net/releases/tag/1.4.21)
+
 #### 1.5.3 May 10 2021 ####
 * Upgraded to [Akka.NET v1.4.19](https://github.com/akkadotnet/akka.net/releases/tag/1.4.19)
 * Upgraded to [Petabridge.Cmd v0.8.5](https://cmd.petabridge.com/articles/RELEASE_NOTES.html#085-may-03-2021)