Added rag_demo md

This jumps in a bit too quickly. I would add a paragraph or two of background saying:

- the current segmentation functions in scikit-image are too fine-grained and fall closer to "superpixel" methods, providing a starting point for segmentation.
- RAGs are a common data structure for many segmentation algorithms.
- You have been working on RAGs for scikit-image and the git head for scikit-image contains your networkx-based implementation from PR (LINK)