Qualify claim of being drop-in replacement to asyncio

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 (https://github.com/MagicStack/uvloop/issues/14, https://github.com/MagicStack/uvloop/issues/536)
• there are a couple of missing features that built-in asyncio has had for years that uvloop does not support, most notably sendfile (https://github.com/MagicStack/uvloop/issues/228, others)

阅读仓库 README 和 uvloop 的开篇文档，然后查看链接的 Windows 和缺失功能 issue，以确认相关注意事项。更新这两项表述，说明 uvloop 在 Windows 上不可用，并且不支持 asyncio 的所有内置功能，包括 sendfile；完成标准是文档不再夸大其 drop-in 兼容性。