Improve formatting and grammar of README

This makes [`README.md`](https://github.com/hchbaw/opp.zsh/blob/master/README.md) use more of the features of Markdown so that it will be easier to read on https://github.com/hchbaw/opp.zsh. I also fix some grammar and capitalization errors.

You will probably need to also edit the comment at the top of [`opp.zsh`](https://github.com/hchbaw/opp.zsh/blob/master/opp.zsh) so it matches this README.