Update authentication.rst

I'm trying to add a link to the blog tutorial's auth section, since there is a bunch of setup work that needs to be done for auth to work, and it's not covered in the main documents (Book nor API) - Having a bit of trouble with the markup however - it's not rendering in github, but I think I've followed the convention - my changes here are on lines 14->21.  Jack Fruh, jack@fruh.net
Merge pull request #1391 from basementjack/patch-1

Update authentication.rst