clarified how to install greadlink (via homebrew) for Mac OS X

Hi there!

Earlier trying to play with dropShell, I got an error as I didn't have ```greadlink``` on my system, so I attempted to comment it out and simply use ```readlink```. Sadly, passing ```-m``` to ```readlink``` on Mac OS X 10.8.5 gives an error, as it's not a valid option, so I set about installing ```greadlink``` but ```brew install greadlink``` came up empty.

After a bit of googling I found out that to install ```greadlink``` on Mac OS X the command is actually:
```brew install coreutils```, so I updated the comment in the code in order to save others a little bit of time if they come across the same issue!

Cheers!