add description for groupware category

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

README.md

@@ -1086,6 +1086,8 @@ Utilities for managing game servers.
 
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+Collaborative software or [groupware](https://en.wikipedia.org/wiki/Collaborative_software) is designed to help people working on a common task to attain their goals. Groupware often regroups multiple services such as file sharing, calendar/events management, address books... in a single, integrated application.
+
 - [BlueMind](https://www.bluemind.net/en/) - Groupware with email, calendar, addressbooks, exchange active sync, exchange MAPI protocol support. ([Source Code](https://forge.bluemind.net/stash/projects/BM/repos/bluemind-public/browse)) `AGPL-3.0` `Java`
 - [Citadel](https://www.citadel.org/) - Groupware including email, calendar/scheduling, address books, forums, mailing lists, IM, wiki and blog engines, RSS aggregation and more. ([Source Code](https://www.citadel.org/source.html)) `GPL-3.0` `C`
 - [Corteza](https://cortezaproject.org) - CRM including a unified workspace, enterprise messaging and a low code environment for rapidly and securely delivering records-based management solutions. ([Demo](https://latest.cortezaproject.org), [Source Code](https://github.com/cortezaproject/corteza-server)) `Apache-2.0` `Go`