Add ref suggegested by @dpshelio
Add ref suggegested by @dpshelio

Co-authored-by: David Pérez-Suárez <83c267b8c29b8a07fd15a211c1b0d4685c796b71@gmail.com>
OpenAstronomy/openastronomy.github.io,OpenAstronomy/openastronomy.github.io,OpenAstronomy/openastronomy.github.io