Update Instructions for .NET Core 3 templates

The instructions for using Serilog contained statements that were no longer valid since .NET Core 3.0 uses the Generic Host and new project templates.

The code sample in the readme and the corresponding statements were updated to resemble these changes.

Author: phlashdev

README.md

@@ -31,7 +31,7 @@ public class Program
         try
         {
             Log.Information("Starting web host");
-            CreateWebHostBuilder(args).Build().Run();
+            CreateHostBuilder(args).Build().Run();
             return 0;
         }
         catch (Exception ex)
@@ -46,21 +46,22 @@ public class Program
     }
 ```
 
-**Then**, add `UseSerilog()` to the web host builder in `BuildWebHost()`.
+**Then**, add `UseSerilog()` to the Generic Host in `CreateHostBuilder()`.
 
 ```csharp        
-    public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
-            WebHost.CreateDefaultBuilder(args)
-                .UseStartup<Startup>()
+    public static IHostBuilder CreateHostBuilder(string[] args) =>
+            Host.CreateDefaultBuilder(args)
+                .ConfigureWebHostDefaults(webBuilder =>
+                {
+                    webBuilder.UseStartup<Startup>();
+                })
                 .UseSerilog(); // <-- Add this line;
 }
 ```
 
 **Finally**, clean up by removing the remaining configuration for the default logger:
 
- * Remove calls to `AddLogging()`
  * Remove the `"Logging"` section from _appsettings.json_ files (this can be replaced with [Serilog configuration](https://github.com/serilog/serilog-settings-configuration) as shown in [the _EarlyInitializationSample_ project](https://github.com/serilog/serilog-aspnetcore/blob/dev/samples/EarlyInitializationSample/Program.cs), if required)
- * Remove `ILoggerFactory` parameters and any `Add*()` calls on the logger factory in _Startup.cs_
  * Remove `UseApplicationInsights()` (this can be replaced with the [Serilog AI sink](https://github.com/serilog/serilog-sinks-applicationinsights), if required)
 
 That's it! With the level bumped up a little you will see log output resembling: