Inform people how to get homebrew-php

When I followed the brew installation instructions, I received the error:

"php-cs-fixer: Missing PHP53, PHP54 or PHP55 from homebrew-php. Please install one of them before continuing"

But, there was no info on how to get that. Adding a section to the README, though ideally this error message would contain it. (I wasn't able to find this string in the source, though.)