Update release notes

.github/workflows/ci.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 env:
   VERSION: "5.0.0.${{github.run_number}}"
-  PACKAGE_SUFFIX: "-beta"
+  PACKAGE_SUFFIX: "-RC"
 
 jobs:
   build:

Source/ReleaseNotes.md

@@ -0,0 +1,10 @@
+**! Read the migration guide (https://github.com/dotnet/MQTTnet/wiki/Upgrading-guide) before migrating to version 5!**
+
+## Changes
+* Memory usage optimizations (thanks to @mregen)
+* Performance optimizations (thanks to @mregen)
+* Removal of no longer supported .NET Frameworks **(BREAKING CHANGE)**
+* Changed code signing and nuget certificate
+* Namespace changes **(BREAKING CHANGE)**
+* Removal of Managed Client **(BREAKING CHANGE)**
+* Client: MQTT 5.0.0 is now the default version when connecting with a server **(BREAKING CHANGE)**