Update README.md fix grammar

Fixed some run-on sentences in the major improvements section.

BEFORE: "Button colors are now complete independent (ex. button-primary) we no longer have classes like <code>button-flat-primary</code> to achieve this you now simply add <code>button-flat button-primary</code>"
AFTER: "Button colors are now complete independent (ex. button-primary). We no longer have classes like <code>button-flat-primary</code>, so to achieve this you now simply add <code>button-flat button-primary</code>"