Update README

Edits sentences for spelling, capitalization, and grammar. Edits sentences for contextual consistency and clarity. Adds some missing descriptions.
Changes '[version]' and paragraph about override options

Changes '[version]' to 'x.y.z' based on feedback from ashp. Adds additional options to override options paragraphs.
Merge pull request #410 from laurenrother/readme

Update README
Moved spec test to acceptance folder
Merge remote-tracking branch 'upstream/master'