To render a graphviz image in a github markdown page, we can merely use the 'Raw' URL to the graphviz text file to a special site which will render it for us.
Simply use the GraphVizRender site https://graphvizrender.herokuapp.com/render to render your *.dot files and embed them as dynamically generated images as so:
![alt text](https://graphvizrender.herokuapp.com/render?"url of your raw file")
Use the link to the Raw version of your *.dot files. For example, copy the link to the raw file and prefix it with the url shown above.
For example, here is a dynamically rendered image of a sample graphviz file in this directory:
Note that if the image doesn't show up immediately that you might need to refresh the page.
Thanks very much to @grahamc for providing this service and for so nicely documenting how to use it in his github repo.
Note that if this service ever stops working, there may be another comparable service.