Ease inclusion of themes, plugins, and uploads

With the boot2docker images compiled with Virtualbox guest additions enabled, [link (medium.com)](https://medium.com/boot2docker-lightweight-linux-for-docker/boot2docker-together-with-virtualbox-guest-additions-da1e3ab2465c), it is possible to include file syncing from the host computer to the boot2docker vm. In my case, this is a guest addition from `/Users/<my-username>/dev/` -> `/Users`, but this can be configured to the users preference.

From there, you can do a `-v "/Users/<path-to-directory-with-optional-subdirectories-themes-plugins-uploads>":/opt` to include these files in a data container, for example `docker run -d -v "/Users/testing":/opt --name theme_vol busybox`. Then when you run the wordpress-stackable container, you would do the `--volumes-from theme_vol` to get those files into the wordpress container, which would then create the symbolic links into the /app directory to allow those files to be edited inside the container live. This seems like a pretty solid wordpress developer environment to me, so I'm opening this pull request to make this change to this image so I don't need to rebuild the image from a fork each time I want to use it. If you all think this is acceptable then I can take a stab at updating the documentation as well.