Add workaround in docs for git bash usage issue

When using boot2docker through git bash, there is a nasty issue when trying to share a folder, due to how git bash parses the URIs. The issue is explained in [issue #12751](https://github.com/docker/docker/issues/12751) but needs more visibility in the docs.

Signed-off-by: Matthieu Larcher <github@ringabell.org>

docs/userguide/dockervolumes.md

@@ -102,6 +102,10 @@ This will mount the host directory, `/src/webapp`, into the container at
 > If the path `/opt/webapp` already exists inside the container's image, its
 > contents will be replaced by the contents of `/src/webapp` on the host to stay
 > consistent with the expected behavior of `mount`
+>
+> When using Boot2Docker on Windows through git bash, there might be an issue with the 
+> way the source directory name is parsed. You can fix it by using a double slash at
+> the beginning of the source directory name as explained in [issue #12751](https://github.com/docker/docker/issues/12751)
 
 This is very useful for testing, for example we can
 mount our source code inside the container and see our application at work as