Adding Amazon Cognito to the list of supported Identity providers (#6039)

Author: Hamza Assyad

src/Identity/README.md

@@ -29,3 +29,4 @@ The previous versions of Identity for MVC5 and lower, previously available on Co
  * [ASP.NET Identity Firebase Provider](https://github.com/aguacongas/Identity.Firebase)
  * [ASP.NET Identity Redis Provider](https://github.com/aguacongas/Identity.Redis)
  * [ASP.NET Identity DocumentDB](https://github.com/FelschR/AspNetCore.Identity.DocumentDB)
+ * [ASP.NET Identity Amazon Cognito Provider](https://github.com/aws/aws-aspnet-cognito-identity-provider)