This page might interest you: http://msdn.microsoft.com/en-us/magazine/dd722812.aspx
You can generate the XML documentation file using either the command-line compiler or through the Visual Studio interface. If you are compiling with the command-line compiler, use options /doc or /doc+. That will generate an XML file by the same name and in the same path as the assembly. To specify a different file name, use /doc:file.
If you are using the Visual Studio interface, there's a setting that controls whether the XML documentation file is generated. To set it, double-click My Project in Solution Explorer to open the Project Designer. Navigate to the Compile tab. Find "Generate XML documentation file" at the bottom of the window, and make sure it is checked. By default this setting is on. It generates an XML file using the same name and path as the assembly.