Documentation makes it too easy to get the wrong name for the overrides directory

Rename README.txt to README.rst so the Readme reads well on github (and, I trust still reads well on pypi too), 

I spent too many hours bashing my head against a wall because the I had set the name of the directory as **template-overrides** instead of **overrides**.  Hopefully making this more readable on github & also adding a "Quick Example" section (or perhaps "Quickstart"?) will stop some others from doing the same.