Improved Xamarin example doc.

Xamarin Studio is no longer preferred for Xamarin development, we should use Visual Studio for Mac.
Also added more info on how to use the nuget package source.

examples/csharp/HelloworldXamarin/README.md

@@ -7,22 +7,25 @@ Support of the Xamarin platform is currently experimental.
 The example depends on experimental Grpc.Core nuget package that hasn't
 been officially released and is only available via the [daily builds](https://packages.grpc.io/)
 source. 
+NOTE: To downlaod the package please manually [download](https://packages.grpc.io/archive/2018/07/a3b54ef90841ec45fe5e28f54245b7944d0904f9-d24c85c7-32ed-4924-b9af-80e7a4aeb34d/index.xml) the .nupkg file into a local directory. Then add a nuget source that points to that directory (That can be done in [Visual Studio](https://docs.microsoft.com/en-us/nuget/tools/package-manager-ui#package-sources) or Visual Studio for Mac via "Configure nuget sources"). After that, nuget will also explore that directory when looking for packages.
+
+
 
 BACKGROUND
 -------------
-The example project supports Xamarin.Android and Xamarin.iOS
+The example project supports `Xamarin.Android` and `Xamarin.iOS`.
 
 For this sample, we've already generated the server and client stubs from [helloworld.proto][].
 
 PREREQUISITES
 -------------
 
-- The latest version Xamarin Studio or Visual Studio 2017 with Xamarin support installed.
+- The latest version Visual Studio for Mac or Visual Studio 2017 with Xamarin support installed.
 
 BUILD
 -------
 
-- Open the `HelloworldXamarin.sln` in Visual Studio (or Xamarin Studio)
+- Open the `HelloworldXamarin.sln` in Visual Studio (or Visual Studio for Mac)
 - Build the solution (Build -> Build All)
 
 Try it!