Update README.md

Make a note that the function under test does not have to be .NET Core 3.0 itself.

README.md

@@ -120,6 +120,8 @@ The key parts to call out here are:
   1. Once the function processing completes after the `CancellationToken` is signalled, the channel reader is read to obtain the `LambdaTestResponse` for the request that was enqueued.
   1. Once this is returned from the channel reader, the response is checked for success using `IsSuccessful` and then the `Content` (which is a `byte[]`) is deserialized into the expected response to be asserted on. Again, you could make your own extensions to deserialize the response content into `string` or objects from JSON.
 
+The library itself targets `netcoreapp3.0` so requires your test project to target at least .NET Core 3.0, but the function you're testing could target a previous version such as .NET Core 2.2.
+
 ### Examples
 
 You can find examples of how to factor your Lambda function and how to test it: