Very pleased with the onebox support. One limitation seems to be that links to readthedocs.org aren’t oneboxed; eg
You have two options here:
implement a custom onebox here: onebox/lib/onebox at master · discourse/onebox · GitHub
(more desirable) add support for opengraph http://ogp.me/ to readthedocs (which would help everywhere) … then you simply whitelist the url.
You should implement OpenGraph at readthedocs.org if you control it, or strongly lobby them to do so.