Update README.md

Examples given in the main page README.md do not work when directly implemented, probably because of some typos. (eg. add() is referenced to as ADD(), subtract() as SUB(), and so on.)
Also, testing it with the add() operator for the given inputs (1011, 11) threw an error:
TypeError: 'int' object has no attribute '__getitem__'
I guess that would be a different bug report, anyway.
However, documentation needs to be double checked.