Using OpenResty for Authentication on Fly

In #278 we talked about authentication being a good example for OpenResty, but it is a little more complex than just “getting started”: https://github.com/zmartzone/lua-resty-openidc

So, I propose we do this article as a standalone topic after the introductory OpenResty one so we can reference it. This is sort of the 200-level content whereas the other one is more 100-level.