add description for digital archiving and preservation category

- ref. https://github.com/awesome-selfhosted/awesome-selfhosted/issues/2984

README.md

@@ -128,6 +128,8 @@ _Related: [Personal Dashboards](#personal-dashboards)_
 
 **[`^        back to top        ^`](#)**
 
+Digital [archiving](https://en.wikipedia.org/wiki/Archival_science) and [preservation](https://en.wikipedia.org/wiki/Digital_preservation) software.
+
 _Related: [Content Management Systems (CMS)](#content-management-systems-cms)_
 
 - [Access to Memory (AtoM)](https://www.accesstomemory.org/) - Web-based, open source application for standards-based archival description and access in a multilingual, multi-repository environment. ([Demo](https://demo.accesstomemory.org/), [Source Code](https://github.com/artefactual/atom)) `AGPL-3.0-only` `PHP`