Tighten up language around badges on README

On https://github.com/badges/shields/issues/82#issuecomment-29879939 we
decided to refer to "badges by Shields." We're not trying to change the
name of the thing itself to a "shield."
"Shields badges" -> "badges" per #105
Merge pull request #105 from badges/readme-shields-to-badges

Tighten up language around badges on README