Add steps for debug Edge Hub using VS in README.md (#314)

Add steps for debug Edge Hub using VS

edge-hub/README.md

@@ -1,2 +1,11 @@
 # Edge Hub
-This project contains the Edge Hub.
+This project contains the Edge Hub.
+
+## How to debug Edge Hub using Visual Studio
+1. Set environment variable `EdgeModuleHubServerCertificateFile` to the path of a SSL Certificate file (e.g. C:\edgeDevice.pfx); for debug purpose, you can [create](https://docs.microsoft.com/en-us/azure/cloud-services/cloud-services-certs-create#create-a-new-self-signed-certificate) and use a self-signed certificate.  Remember to restart Visual Studio to take effect.
+2. Update following values in appsettings_hub.json in Microsoft.Azure.Devices.Edge.Hub.Service project. 
+  * `IotHubConnectionString` - Edge Hub module connection string; set it as edge device connection string appending with "ModuleId=$edgeHub".
+  * `configSource` - Set it as `twin` to read confirm from twin, or `local` to read Edge Hub config from configuration file.
+3. Set Microsoft.Azure.Devices.Edge.Hub.Service as startup project in Visual Studio.
+4. Make sure to rebuild the solution.
+5. You can start debugging Edge Hub by hit F5 in Visual Studio.