set up https for custom domain at Read The Docs

[ryanpitts]

they support HTTPS on custom domains now! https://docs.readthedocs.io/en/latest/alternate_domains.html#cname-ssl

[ryanpitts]

OK, this works: https://securitytraining.opennews.org/

Currently there is NO automatic redirect, however, from http://securitytraining.opennews.org/. RTD is actively working on this, so just noting here for now.