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)

Lee el README del repositorio y la documentación inicial de uvloop; después, revisa las issues enlazadas sobre Windows y las funciones que faltan para confirmar las salvedades. Actualiza ambas afirmaciones para explicar que uvloop no está disponible en Windows y no admite todas las funciones integradas de asyncio, incluido sendfile; la tarea estará terminada cuando la documentación ya no exagere la compatibilidad drop-in.