Add some information in src/desktop/README.md

src/desktop/README.md

@@ -2,3 +2,24 @@
 *Copyright (C) 2017-2018 David Capello*
 
 > Distributed under [MIT license](LICENSE.txt)
+
+## Windows
+
+On Windows we have to create a COM server in a DLL to provide
+thumbnails. The DLL must provide an
+[IThumbnailProvider](https://msdn.microsoft.com/en-us/library/windows/desktop/bb774614.aspx)
+implementation. Our implementation is in
+[desktop::ThumbnailHandler](win/thumbnail_handler.h) class, the most
+interesting member function is
+[ThumbnailHandler::GetThumbnail()](win/thumbnail_handler.cpp),
+which should return a `HBITMAP` of the thumbnail.
+
+### Registering the DLL
+
+If you distribute your app in an installer remember to use
+`regsvr32.exe` to register your DLL which will call your DLL's
+[DllRegisterServer()](win/dllmain.cpp), a function that must
+create registry keys to associate your file type extension with
+your thumbnail handler.
+
+More information [in the MSDN](https://msdn.microsoft.com/en-us/library/windows/desktop/cc144118.aspx).