fix README formatting

detailed list of changes:

- fix code formatting of `index.less` – GitHub-Flavored Markdown is not supported in repository files
- add inline code formatting
- change the “Version”: “1.0” section to a prettier and more-semantic `<dl>` definition list
- since all headings have the wrong number of underlines, just change to three underlines, so they don’t need to be maintained in the future
- fix grammar, capitalization, and typography
- make “LESS” a link in case someone stumbles on this project without knowing what it is
- add blank lines between sections to make them more visually distinguishable