Add versioning hints to the readme

Author: ChrisDoernen

README.md

@@ -8,6 +8,13 @@ Xunit does not support any built-in dependency injection features, therefore dev
 
 This library brings in Microsoft's dependency injection container to Xunit by leveraging Xunit's fixture.
 
+## Important: xUnit versions
+
+For xUnit packages use Xunit.Microsoft.DependencyInjection up to 9.x.x
+For xUnit.v3 packages use Xunit.Microsoft.DependencyInjection from 10.x.x
+
+Also check the [migration guide](https://xunit.net/docs/getting-started/v3/migration) from xUnit for test authors.
+
 ## Getting started
 
 ### Nuget package