Updated mac.md to include workaround for volumes

Updated the documentation to include an explanation for handling volume support in Mac OS X.  This was quite a hinderance to a first-timer, and I feel it is exceedingly important.

Credits to:
https://coderwall.com/p/fvfjyg
https://medium.com/boot2docker-lightweight-linux-for-docker/boot2docker-together-with-virtualbox-guest-additions-da1e3ab2465c