Update README.rst

Hi, just starting to install bc-bio using docker. So as not to disrupt any of my current set up I'm running the installation inside a virtual machine (does this defeat the object of docker? I'm still learning!!). I ran into a problem where I had not setup my VM with enough free disk space so thought it might be useful to let people know how much disk space is required?

Thanks for bc-bio and all your hard work, looking forward to trying it out!

Chris