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)

閱讀儲存庫的 README 和 uvloop 的開頭文件，然後檢視連結的 Windows 與缺少功能 issue，以確認相關注意事項。更新這兩項說法，說明 uvloop 無法在 Windows 上使用，且不支援 asyncio 的所有內建功能，包括 sendfile；完成條件是文件不再誇大其 drop-in 相容性。