Update readme

- Sync it with the reworkcss/css readme.
- Put the "Built with rework" section last.
- Remove the "descriptions" of the "Built with rework" entries.
- Explicitly say that `options` are passed to `css`.
- Document the `sourcemapAsObject` option.
- Minor whitespace and punctuation fixes.
Merge pull request #168 from lydell/master

Update readme