Install file updated with boost instructions for mac users

I have added an instruction for Mac users based on the thread titled "Cannot install, "undefined symbols for architecture"" at https://github.com/moses-smt/mgiza/issues/1

It took me a couple of hours of looking around before I found the thread. Hopefully, other users won't have the same problem.

I hope you find it useful.

Thanks,
Prateek Goel