docs: Add v3 -> v4 migration steps (#2854)

v4-migration-guide

docs/migration-guide.md

@@ -8,6 +8,14 @@ custom_edit_url: https://github.com/stryker-mutator/stryker-net/edit/master/docs
 
 This guide instructs how to update major versions of Stryker.NET.
 
+### ⏭ Updated runtime
+
+The .NET runtime requirement for Stryker.NET has been changed. Dotnet 8 runtime is now required.
+
+Please [download and install the dotnet 8 runtime]([https://dotnet.microsoft.com/download/dotnet/5.0](https://dotnet.microsoft.com/en-us/download/dotnet/8.0)) and update your pipeline have the dotnet 8 runtime available.
+
+_Note it is not neccesary to target dotnet 8 in your projects. Dotnet 8 is a runtime requirement for stryker, we will still compile your project to your existing target frameworks._
+
 ## V2.x --> V3.x
 
 ### :pushpin: Baseline