Expand documentation for `with_tls()`

[gavinmorrow]

Hello! Today I was trying to serve a website over https using mist.with_tls(), and had a hard time figuring out what formats the certfile and keyfile should be in.

It may just be me (since I know essentially nothing about certificates), but maybe it would help to put the expected type of each file in the docs or to provide a working example w/ commands to generate a cert? Currently the with_tls() documentation just says "certificate and key files," and specifies nothing else.

(I eventually figured it out after reading the glisten readme and seeing .crt and .key, when led me to a helpful serverfault question.)

(Also, I originally created a discord thread in the gleam discord.)