Update RELEASE_NOTES.md for 1.5.18 release (#232)

Author: Arkatufus

RELEASE_NOTES.md

@@ -1,3 +1,11 @@
+#### 1.5.18 April 12 2024
+
+* [Updated Akka.NET to 1.5.18](https://github.com/akkadotnet/akka.net/releases/tag/1.5.18)
+* Dependency updates:
+  * [Update System.CodeDom to 8.0.0](https://github.com/akkadotnet/Akka.MultiNodeTestRunner/pull/217)
+  * [Update TeamCity.ServiceMessages to 4.1.1](https://github.com/akkadotnet/Akka.MultiNodeTestRunner/pull/205)
+  * [Update xUnit to 2.5.3](https://github.com/akkadotnet/Akka.MultiNodeTestRunner/pull/228)
+
 #### 1.5.13 September 19 2023
 
 * [Updated Akka.NET to 1.5.13](https://github.com/akkadotnet/akka.net/releases/tag/1.5.13)