Update README on configuring servers for sendfile support

[sideffect0]

The Django app uses Sendfile supports from Nginx, Apache, Caddy to Proxygen.
a readme section on how to configure server for Sendfile support is a good addition.

[Evalle]

hey, @sideffect0 may I take a look at it?

[sideffect0]

@Evalle sure!

[sideffect0]

@Evalle any progress?