Add requirements.txt to enable using binder

[jamesmyatt]

Live notebooks: https://mybinder.org/v2/gh/jamesmyatt/effective-pandas/master

[TomAugspurger]

Cool, thanks.

Do you know how well the binder instances handle downloading the data? I was kinda relying on them being downloaded once and cached for re-use across notebooks.

[jamesmyatt]

The container is persistent until it's been inactive for too long. So data downloaded by one notebook will be available to the others.

[jamesmyatt]

I'm pretty sure that I'm missing some requirements, but it'll be easy to check once I can run the notebooks directly via binder.