Added a warning about serializing data with JSON cookie jars [skip ci]

A quick fix for #14409. Not super happy with this yet, so I wanted to get some feedback. Also an excuse to try [GitHub Selfies](http://blog.travisthieman.com/github-selfies/)!

Is the full example helpful/necessary? Should I explicitly mention session/flash as well?

![selfie-1](http://i.imgur.com/7Ulgxcw.png)