Update documentation on key and cert

[hirasso]

Hi there! I was struggling today to get the key and cert options to work with tiny-lr. Following the available documentation I thought the both options would need a path to the files, but it turns out that they expect the actual file contents.

Maybe this should be mentioned in the documentation, otherwise I can imagine many people could have problems getting it to work, just like me today. Just a link to the official documentation would be great, already: https://nodejs.org/api/https.html#https_https_createserver_options_requestlistener

Are you accepting pull requests? Then I would offer my help and add the documentation for this behavior myself.