Document and expose content_hash

[olivierdalang]

Hi ! Thanks for the great package !!

The mode has a content_hash column, however it is not exactly clear what kind of hash this is.

From the code, it looks like it could be a sha256 hash hexdigest, but there also seem to be some custom logic which tends to indicate it's not a standard hash.

Having a documented (or even better customizable) hash function would allow to reuse this for various use case. It would also be great to have an example on how to use this hash.