Add state diagrams to elevator example readme file

[hlovdal]

Hi. I have created two state diagrams for the readme file.

They are graphical png files though if you actually prefer ascii-art, it is possible to generate at https://dot-to-ascii.ggerganov.com/.

[hlovdal]

You can preview at https://github.com/hlovdal/tinyfsm/tree/elevator_doc/examples/elevator.