Filled out README for pandas 101 session

While I added a brief introduction to the `pandas` session of the RST repo, I also observed that the `ipython`, `pandas`, and `matplotlib` libraries have progressed quite a bit since then. In one case, the `pandas` library raises warnings for actions that didn't before. I'll add another story to possibly update these in the future.

I also removed all the output cells, for easier version control of the repo.