Document breaking changes on CSS classes

In the latest commits there were some breaking changes on the CSS styles for cursors and highlights. These styles must be properly documented. 

- `atHighlight` -> `at-highlight`
- `cursors` -> `at-cursors`
- `selectionWrapper` -> `at-selection`
- `barCursor` -> `at-cursor-bar`
- `beatCursor` -> `at-cursor-beat`

e2658270de090fae32d87ebdb8dbb124a4d4c8aa