Added information about accessing host directory on Windows.

Signed-off-by: Tibor Vass <tibor@docker.com>

docs/sources/userguide/dockerimages.md

@@ -242,6 +242,9 @@ Let's create a directory and a `Dockerfile` first.
     $ cd sinatra
     $ touch Dockerfile
 
+If you are using Boot2Docker on Windows, you may access your host
+directory by `cd` to `/c/Users/your_user_name`.
+
 Each instruction creates a new layer of the image. Let's look at a simple
 example now for building our own Sinatra image for our development team.