Update RELEASE_NOTES.md (#6741)

Aaronontheweb · web-flow · commit f7a59776fc41 · 2023-05-08T20:31:21.000Z
Added release notes for 1.5.7-beta1, so we can do a pre-release of `Akka.Delivery`
diff --git a/RELEASE_NOTES.md b/RELEASE_NOTES.md
@@ -1,3 +1,7 @@
+#### 1.5.7-beta1 May 9th 2023 ####
+
+*Placeholder for nightlies*
+
 #### 1.5.6 May 8th 2023 ####
 
 Version 1.5.6 is a patch with a few minor bug fix
