Updated README with examples of equivalent normal Swift code
Updated README with examples of equivalent normal Swift code

After thinking about this for a bit, and doing a bit of coding in a project in which I don't have NilCoalescingAssignmentOperators pulled it, it made sense to show potential users the code they're currently writing, to make the usefulness of `??=` and `=??` click.
capnslipp/NilCoalescingAssignmentOperators,capnslipp/NilCoalescingAssignmentOperators