Missing documentation: deploying Django in production behind a proxy.

[Klaas van Schelven]

When Django is deployed behind a proxy (with https), various settings _must_ be set for your site to work correctly. Currently, there is no single place where these settings are documented. That is, they are mentioned in the general docs for settings (with a warning "only do this when behind a proxy) but there is no page "how to set up Django behind a proxy", which means to do this you must now puzzle various Stack Overflow / blog posts together.

The variables that I could find are SECURE_PROXY_SSL_HEADER, USE_X_FORWARDED_HOST and USE_X_FORWARDED_PORT (if the port is not included in the host), but there may be more.

[Sarah Boyce]

There's need talk that the Django docs should better help with deployment, so I would happily review PRs on this.