aiofile backend

[williamjamir]

First of all, thanks for this lib! You made an awesome job porting the pathlib interface to async =)

I have a question about your readme, under the "Implementation" section there is a paragraph that mentions the following.

aiopath is typed with Python type annotations, and if using the aiofile back end, it takes advantage of libaio for async I/O on Linux.

I didn't get the part "if using the aiofile", is there any other option besides aiofile?