Update README.md (#1817)

Author: JamesNK

src/Grpc.AspNetCore.Web/README.md

@@ -31,6 +31,16 @@ app.UseEndpoints(endpoints =>
 });
 ```
 
+### gRPC-Web and streaming
+
+Traditional gRPC over HTTP/2 supports streaming in all directions. gRPC-Web offers limited support for streaming:
+
+* gRPC-Web browser clients don't support calling client streaming and bidirectional streaming methods.
+* gRPC-Web .NET clients don't support calling client streaming and bidirectional streaming methods over HTTP/1.1.
+* ASP.NET Core gRPC services hosted on Azure App Service and IIS don't support bidirectional streaming.
+
+When using gRPC-Web, we only recommend the use of unary methods and server streaming methods.
+
 ## Links
 
 * [Documentation](https://docs.microsoft.com/aspnet/core/grpc/browser)