set up commenting feature

Timon has Hypothesis set up---is that what was used for Hydrology book?

If that is the esiest way to go can you set it up, and add some documentation to the manual or the readme in this book?

this is the text on Timon's front page too:

The bar on the right of this page opens [Hypothesis](https://web.hypothes.is/), a service that allows you to add comments to pieces of text in this book.