Add an overview to the extension

[rufuspollock]

e.g. could reuse material from https://github.com/datopian/ckanext-versioning/blob/master/README.md

CKAN + data versioning 🚀. This CKAN extension adds a full data versioning capability to [CKAN][] including:

* Metadata and data is **revisioned** so that all updates create new revision and old versions of the metadata and data are accessible
* Create and manage **releases** - named labels plus a description for a specific revision of a dataset, e.g. "v1.0". 
  These are similar in concept to VCS tags. 
* Diffs, reverting etc

For more background see https://tech.datopian.com/versioning/

[CKAN]: https://ckan.org/