Fixup README file

Changes:

* Rename ipynb files
* Add links and explanations for ipynb files
* Explain how the talk was given
* Note license

Thoughts on this?

Is GPL the appropriate license for this?  It seems like a Creative Commons license may be more suitable.
This is my justification for linking to nbviewer only:

The files are not very human readable.  If you're checking out the repo, the links don't help much.  If you're looking on Github, the links should probably go somewhere readable.