Add documentation for routing WebSockets through a reverse proxy

Summary: This is a little rough and should be considered an "advanced" option. Having said that, this works well in my install and I imagine that other installs will find this beneficial.

Test Plan: Eyeball it.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11293