Accept alternate readthedocs.org domain names

[paternal]

According to your code, the only accepted domain name for documentation is http://readthedocs.org. However, other valid domain names are available (see the readthedocs documentation): http://rtfd.io, http://rtfd.org, http://readthedocs.io.

Shouldn't these be accepted as well?

Regards,
-- Louis

[regebro]

After considering this back and forth I decided I shouldn't recommend any places to host documentation.

This is fixed in master, the check is gone.