pandas-guide: Add py-requirements and Makefile to make contribution easy

[dukebody]

I had two issues when updating the guide, so I decided to make it a bit easier for future contributors:

1. I had a different version of sphinx and I had to update to the last version, so I created a requirements.txt file.
2. I didn't remember the command to update the guide, so I created a Makefile. To use it, just make html in Linux.

[datapythonista]

We'll be moving all this to pandas at some point. But useful until then.