Improve formatting and grammar of README

This makes 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.
Update the docs; no code changes

Simply delete the contents which I feel it could be deleted.

Signed-off-by: Takeshi Banse <takebi@laafc.net>
Opp 0.0.8

Signed-off-by: Takeshi Banse <takebi@laafc.net>