Missing type hints & readme updates

As of v0.8.1, constructor of `ResourceOptions` does not have type hints, which causes an error, when type checking consumer code with MyPy in strict mode. A few functions & methods don't have the same issue, but could use type annotations, e.g.: `custom_cors` and `CorsConfig.add`.

There's also a couple of things that could be updated in the readme file:
1) `ResourceOptions` class is missing `s` at the end in one the examples;
2) `@asyncio.coroutine` could be replaced with `async` keyword.