Check out the extended guidelines:
http://www.constant-content.com/about/e ... eferencing sources
Referencing Sources
We do not accept content with links or website addresses. However, if you want to reference websites, you may do so by dropping the
http://www. prefix from the address and removing hyperlinks.
You may use your preferred style for referencing print sources.
From the FAQ
Submissions and Rejections
My article was rejected for containing website addresses or links. Why?
Buyers prefer articles without links because they want readers to stay on their websites. Web addresses or links can lead visitors away. Links are only permissible if the requester specifically asks for it. Make sure to add this information into the short summary so that the editor can see it.