From the announcement <http://blog.readthedocs.com/securing-subdomains/>:
> Starting today, Read the Docs will start hosting projects from
> subdomains on the domain readthedocs.io, instead of on readthedocs.org.
[...]
> Projects will automatically be redirected, and this redirect will
> remain in place for the foreseeable future. Still, you should plan on
> updating links to your documentation after the new domain goes live.
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