Update a link to Django documentation
Update a link to Django documentation

Documentation for unsupported versions of Django has been withdrawn from
the official Django website, but it is available on Read the Docs.

Fixes #235.

erlydtl/erlydtl,campanja-forks/erlydtl,campanja-forks/erlydtl,kivra/erlydtl,erlydtl/erlydtl,erlydtl/erlydtl,campanja-forks/erlydtl,kivra/erlydtl,kivra/erlydtl