Update README.md

Homebrew recently booted a bunch of formulas into their science repository, without telling anyone or providing deprecation warnings, and VIPS has been included in the move. 

<rant>
I opened up a ticket about this with Homebrew as the change has frustrated me a great deal and I think makes it (homebrew) substantially less easy to use. 
</rant>

Regardless, for the mean time, this is how to install VIPS via homebrew now.