Update README.md

This is a quick documentation update which stopped me from using FPM. I had to look up what mkmf was and how to install it before i was able to built a package with FPM. Hopefully it should stop the below kinds of issues being logged as well.

https://github.com/jordansissel/fpm/issues/366
https://github.com/jordansissel/fpm/issues/666
https://github.com/jordansissel/fpm/issues/282