Link to integration tests readme (#21329)

Author: miloush

docs/build.md

@@ -74,6 +74,8 @@ Or if you need to create nuget packages as well (it will compile and run tests a
 Alternatively, you can run nuke build directly without installing Nuke global tool:
 `dotnet run --project nukebuild/_build.csproj -- --configuration Debug`
 
+For integration tests, see [readme](https://github.com/AvaloniaUI/Avalonia/tree/master/tests/Avalonia.IntegrationTests.Appium#readme) next to the project.
+
 # Linux/macOS
 
 It's *not* possible to build the *whole* project on Linux/macOS. You can only build the subset targeting .NET Standard and .NET Core (which is, however, sufficient to get UI working on Linux/macOS). If you want to something that involves changing platform-specific APIs you'll need a Windows machine.