Added sample to set Android Tag field (#8)

Author: Petrol-

README.md

@@ -24,6 +24,7 @@ When using Xamarin.Android
 ```csharp
 Log.Logger = new LoggerConfiguration()
     .WriteTo.AndroidLog()
+    .Enrich.WithProperty(Constants.SourceContextPropertyName, "MyCustomTag") //Sets the Tag field.
     .CreateLogger();
 ```