Update cache-web-app-howto.md

Author: CawaMS

articles/azure-cache-for-redis/cache-web-app-howto.md

@@ -21,6 +21,7 @@ Clone the repo [https://github.com/Azure-Samples/azure-cache-redis-samples/tree/
 
 - Azure subscription - [create one for free](https://azure.microsoft.com/free/dotnet)
 - [Visual Studio 2019](https://www.visualstudio.com/downloads/) with the **ASP.NET and web development** and **Azure development** workloads.
+- [.NET Framework 4 or higher](https://dotnet.microsoft.com/download/dotnet-framework) is required by the StackExchange.Redis client.
 
 ::: zone pivot="azure-managed-redis"