Created an Example 3

Hello, I've been working on what I think would be a good example plugin for the GREET API, and it's just a simple demonstration of how to pull the raw data out of GREET and use it to do calculations and things separate from the GREET model itself. For my work I'm using a certain amount of GREET data, but totally ignoring the rest of it, so I thought that maybe this would be something another developer who was working on something like this could relate to, and find useful. I've also included some additions to the README, namely the presence of the instructions that can be found on the API page on the GREET website.