It's copy-pasted from [the standard install instructions for bitters](https://github.com/thoughtbot/bitters/blob/master/README.md#install-instructions).

I believe that this is only here because refills relies on bitters. The refills readme already has a link through to the install instructions for bitters (ie. "Install Bitters"), and so anyone installing bitters with those instructions would already have done this. If someone changed the install instructions for bitters then the instructions for refills would become out-of-date.