Added Stashbox to the external container list (#492)

README.md

@@ -14,6 +14,7 @@ Contains common DI abstractions that ASP.NET Core and Entity Framework Core use.
 * [**Grace**](https://www.nuget.org/packages/Grace.DependencyInjection.Extensions)
 * [**LightInject**](https://github.com/seesharper/LightInject.Microsoft.DependencyInjection)
 * [**StructureMap**](https://github.com/structuremap/StructureMap.Microsoft.DependencyInjection)
+* [**Stashbox**](https://github.com/z4kn4fein/stashbox-extensions-dependencyinjection)
 
 
 This project is part of ASP.NET Core. You can find samples, documentation and getting started instructions for ASP.NET Core at the [Home](https://github.com/aspnet/home) repo.