Qualify claim of being drop-in replacement to asyncio

[FeldrinH]

The readme of this repo as well as the start of the uvloop documentation claims that uvloop is a "drop-in replacement of the built-in asyncio event loop". I know of at least two important caveats to this claim that I think should be mentioned somewhere in the readme/documentation:

• uvloop does not work on Windows, whereas built-in asyncio does (#14, #536)
• there are a couple of missing features that built-in asyncio has had for years that uvloop does not support, most notably sendfile (#228, others)