Adding a GroupBy specification example

src/ApplicationCore/Interfaces/ISpecification.cs

@@ -11,6 +11,7 @@ namespace Microsoft.eShopWeb.ApplicationCore.Interfaces
         List<string> IncludeStrings { get; }
         Expression<Func<T, object>> OrderBy { get; }
         Expression<Func<T, object>> OrderByDescending { get; }
+        Expression<Func<T, object>> GroupBy { get; }
 
         int Take { get; }
         int Skip { get; }