Update documentation

[dmitrypolo]

The documentation (read-the-docs and README) has all the examples using asyncio.coroutine to decorate functions to indicate they are coroutines. However, the docs for Sphinx appear to have the proper format.

The README and read-the-docs should be updated to reflect these changes, especially since the library only supports Python 3.6+.

[jonathanslenders]

Hi @dmitrypolo, thanks for reporting. Would you mind creating a little PR for this? Then I'll merge it.