Not sure if this should actually be a link pointing to the
`persistence.rs` file but following the conventions of the rest of the
file, change `persistence.rs` reference to simply be a file name
mention.
## Problem
External contributors need information on how to use the storage
controller.
## Summary of changes
- Background content on what the storage controller is.
- Deployment information on how to use it.
This is not super-detailed, but should be enough for a well motivated
third party to get started, with an occasional peek at the code.
