add watch command in app

Author: rmannibucau

src/docfx/Program.cs

@@ -23,6 +23,7 @@ internal static int Main(string[] args)
 
             config.AddCommand<InitCommand>("init");
             config.AddCommand<BuildCommand>("build");
+            config.AddCommand<WatchCommand>("watch");
             config.AddCommand<MetadataCommand>("metadata");
             config.AddCommand<ServeCommand>("serve");
             config.AddCommand<PdfCommand>("pdf");