Update RELEASE_NOTES.md for v0.4.3 release (#104)

RELEASE_NOTES.md

@@ -1,3 +1,27 @@
+## [0.4.3] / 9 September 2022
+- [Update Akka.NET from 1.4.40 to 1.4.41](https://github.com/akkadotnet/akka.net/releases/tag/1.4.41)
+- [Cluster.Hosting: Add split-brain resolver support](https://github.com/akkadotnet/Akka.Hosting/pull/95)
+- [Hosting: Add `WithExtension<T>()` extension method](https://github.com/akkadotnet/Akka.Hosting/pull/97)
+
+__WithExtension<T>()__
+
+`AkkaConfigurationBuilder.WithExtension<T>()` works similarly to `AkkaConfigurationBuilder.WithExtensions()` and is used to configure the `akka.extensions` HOCON settings. The difference is that it is statically typed to only accept classes that extends the `IExtensionId` interface.
+
+This pull request also adds a validation code to the `AkkaConfigurationBuilder.WithExtensions()` method to make sure that all the types passed in actually extends the `IExtensionId` interface. The method will throw a `ConfigurationException` exception if one of the types did not extend `IExtensionId` or if they are abstract or static class types.
+
+Example:
+```csharp
+// Starts distributed pub-sub, cluster metrics, and cluster bootstrap extensions at start-up
+builder
+    .WithExtension<DistributedPubSubExtensionProvider>()
+    .WithExtension<ClusterMetricsExtensionProvider>()
+    .WithExtension<ClusterBootstrapProvider>();
+```
+
+__Clustering split-brain resolver support__
+
+The split-brain resolver can now be set using the second parameter named `sbrOption` in the `.WithClustering()` extension method. You can read more about this in the [documentation](https://github.com/akkadotnet/Akka.Hosting/tree/dev/src/Akka.Cluster.Hosting#configure-a-cluster-with-split-brain-resolver-sbr).
+
 ## [0.4.2] / 11 August 2022
 - [Update Akka.NET from 1.4.39 to 1.4.40](https://github.com/akkadotnet/akka.net/releases/tag/1.4.40)
 - [Add `WithExtensions()` method](https://github.com/akkadotnet/Akka.Hosting/pull/92)