Adding reference to IISIntegration in the IIS docs (#1609)

* Adding reference to IISIntegration in the IIS docs

The application is required to inject the IISIntegration module. This is not obvious for an ASP.Net Core beginner.
Added the error to the troubleshooting guide.

* Addressing PR comments.

* Addressing PR comments (2)

aspnet/publishing/iis.rst

@@ -54,6 +54,22 @@ For more information on the ASP.NET Core Module, including configuration of the
 Application configuration
 -------------------------
 
+Enabling the `IISIntegration` components
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Include a dependency on the `Microsoft.AspNetCore.Server.IISIntegration` package in the application dependencies. Incorporate IIS Integration middleware into the application by adding the `.UseIISIntegration()` extension method to `WebHostBuilder()`.
+
+.. code-block:: csharp
+
+  var host = new WebHostBuilder()
+    .UseKestrel()
+    .UseContentRoot(Directory.GetCurrentDirectory())
+    .UseIISIntegration()
+    .UseStartup<Startup>()
+    .Build();
+
+Note that code calling `.UseIISIntegration()` does not affect code portability.
+
 Setting `IISOptions` for the `IISIntegration` service
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -291,6 +307,18 @@ Troubleshooting
 
 - Confirm that the Application Pool is not in the `Stopped` state.
 
+IISIntegration middleware not implemented
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+- **Browser:** HTTP Error 502.5 - Process Failure
+- **Application Log (Portable app):** Process was created with commandline '"dotnet" ...' but either crashed or did not reponse within given time or did not listen on the given port 'PORT', ErrorCode = '0x800705b4'
+- **Application Log (Self-contained app)**: Process was created with commandline '"PATH\my_application.exe" ' but either crashed or did not reponse within given time or did not listen on the given port 'PORT', ErrorCode = '0x800705b4'
+- **ASP.NET Core Module Log:** Log file created and shows normal operation.
+
+Troubleshooting
+
+- Confirm that you have correctly referenced the IIS Integration middleware by calling the `.UseIISIntegration()` method of the application's `WebHostBuilder()`.
+
 Additional resources
 --------------------