Add note about license key must not be blank #900
Description
X-Ref AvaloniaUI/Accelerate#21 (comment)
In https://docs.avaloniaui.net/accelerate/installation it shows how to specify the license key in the project file.
To help avoid future confusion, it seems worth adding a note that this value must not be left blank, or the project will not be able to build or open in Visual Studio.