Add description of specific data cleanup tasks
Add description of specific data cleanup tasks

Having a description of some of the specific tasks that the retriever
can handle, as well as a couple of examples of exactly what it does to
particular datasets should help make clear exactly what the software is
doing and why it is useful to users.

This change was motivated by this conversation with @glaroc on Twitter:
https://twitter.com/GuillaumeLarocq/status/704351892659412992

goelakash/retriever,henrykironde/deletedret,goelakash/retriever,henrykironde/deletedret