Document that Rendertron is not supposed to be run with untrusted web data

[AVGP]

Rendertron should be configured with an allow-list and behind a reverse proxy, rather than exposed to the open internet.

This should be reflected in the readme as well as the documentation.

[Patil2099]

I would love to submit a pr for this.

[AVGP]

In general, as we're deprecating the project, you should look into alternative approaches to rendering on the web.