Add flowchart markdown using mermaid

[davidyaha]

This would be a great addition to to explain code.
Even though you would not have the same rendering options on github.

http://knsv.github.io/mermaid/flowchart.html

[stubailo]

I think there are a lot of things we would probably add before adding additional markdown features, but if you would like to submit a PR to add this support, I'll happily merge it!