Add hash length to README.md

This information is useful for people creating schemas which store bcrypt hashes. According to [this StackOverflow question](http://stackoverflow.com/questions/5881169/what-column-type-length-should-i-use-for-storing-a-bcrypt-hashed-password-in-a-d) hashes can also be 59 characters long but every hash I've generated with this library starts with `$2a$` so I assume it's safe to assume 60 characters.