Add README file to rake-fast plugin

Following the discussion in https://github.com/robbyrussell/oh-my-zsh/issues/2640#issuecomment-38394258, I moved the instructions out of the plugin to a dedicated README.md file which renders nicely when you browse the directory on GitHub: https://github.com/KevinBongart/oh-my-zsh/tree/add-readme-to-rake-fast/plugins/rake-fast