Update RELEASE_NOTES.md for 1.4.9 release (#6840)

* Update RELEASE_NOTES.md for 1.4.9 release

* Update date

RELEASE_NOTES.md

@@ -1,6 +1,46 @@
-#### 1.5.9 June 15th 2023 ####
+#### 1.5.9 July 18th 2023 ####
 
-*Placeholder for nightlies*
+Akka.NET v1.5.9 is a maintenance release that introduces some performance improvements and internal code cleanup/modernization.
+
+__Changes:__
+* [Remoting: Make transport adapter component public](https://github.com/akkadotnet/akka.net/pull/6838)
+
+__Improvements:__
+* [Memory optimization, use `Array.Empty` instead of creating empty arrays](https://github.com/akkadotnet/akka.net/pull/6801)
+* [Remoting: Log all wrapped message layers during errors](https://github.com/akkadotnet/akka.net/pull/6818)
+* [Port #6805 and #6807, Improve Stream and Pattern.Backoff instance creation time performance](https://github.com/akkadotnet/akka.net/pull/6821)
+* [DData: Harden LWWDictionary serialization null check](https://github.com/akkadotnet/akka.net/pull/6837)
+
+__Code modernization:__ 
+* [Use C# 9.0 target-typed new()](https://github.com/akkadotnet/akka.net/pull/6798)
+* [Use C# 8.0 null-coalescing operator](https://github.com/akkadotnet/akka.net/pull/6814)
+
+__Update dependency versions:__
+* [Bump Google.Protobuf to 3.23.4](https://github.com/akkadotnet/akka.net/pull/6826)
+* [Bump Akka.MultiNode.TestAdapter to 1.5.8](https://github.com/akkadotnet/akka.net/pull/6802)
+* [Bump Microsoft.Data.SQLite to 7.0.9](https://github.com/akkadotnet/akka.net/pull/6835)
+* [Bump Microsoft.Extensions.ObjectPool to 7.0.8](https://github.com/akkadotnet/akka.net/pull/6813)
+* [Bump Xunit to 2.5.0](https://github.com/akkadotnet/akka.net/pull/6825)
+
+__Akka.TestKit.Xunit Changes__
+
+Due to breaking API change in Xunit 2.5.0, updating to Akka.NET 1.5.9 might break your unit tests. Some of the breaking change that we've noticed are:
+
+* `AkkaEqualException` constructor has been changed due to changes in Xunit API. If you're using this class, please use the `AkkaEqualException.ForMismatchedValues()` static method instead of using the constructor.
+* Testing for exception types by calling async code inside a sync delegate will not unwrap the `AggregateException` thrown. Either use async all the way or manually unwrap the exception.
+* Xunit `Asset.Equal()` does not automatically check for collection item equality anymore, that means doing `Assert.Equal()` between two dictionary or list would not work anymore.
+* Some Xunit classes have been changed from public to private. If you're using these classes, you will need to refactor your code.
+* __FsCheck.Xunit:__ Xunit Roslyn analyzer has become a bit too overzealous and insists that all unit test method can only return either void or Task and will raise a compilation error if you tried to return anything else. If you're using `FsCheck.Xunit`, you will need to use a pragma to disable this check: `#pragma warning disable xUnit1028`.
+
+If you want to see the [full set of changes made in Akka.NET v1.5.9, click here](https://github.com/akkadotnet/akka.net/milestone/91?closed=1).
+
+| COMMITS | LOC+ | LOC- | AUTHOR              |
+|---------|------|------|---------------------|
+| 12      | 171  | 155  | dependabot[bot]     |
+| 7       | 466  | 165  | Aaron Stannard      |
+| 4       | 1648 | 1725 | Simon Cropp         |
+| 1       | 9    | 4    | Gregorius Soedharmo |
+| 1       | 7    | 1    | Michael Buck        |
 
 #### 1.5.8 June 15th 2023 ####