Adds CSS 2.0 whitepaper to README.md

Author: aljoshakoecher

README.md

@@ -1,5 +1,5 @@
 # CSS-Ontology
-An ontology containing the definitions of the Capability, Skill and Service (CSS) reference model of Plattform Industrie 4.0. The model is presented in this [whitepaper](https://www.plattform-i40.de/IP/Redaktion/EN/Downloads/Publikation/CapabilitiesSkillsServices.html). There is also a scientific publication about the CSS reference model in case you want to cite this work: https://www.degruyter.com/document/doi/10.1515/auto-2022-0117/html
+An ontology containing the definitions of the Capability, Skill and Service (CSS) reference model of Plattform Industrie 4.0. The model is presented in this [whitepaper](https://www.plattform-i40.de/IP/Redaktion/EN/Downloads/Publikation/CapabilitiesSkillsServices.html). Extensions were later added in a [follow-up whitepaper](https://www.plattform-i40.de/IP/Redaktion/DE/Downloads/Publikation/2025-i40-capabilities.pdf?__blob=publicationFile&v=5). There is also a scientific publication about the CSS reference model in case you want to cite this work: https://www.degruyter.com/document/doi/10.1515/auto-2022-0117/html.
 
 ## Overview
 This is an ontology implementation of the CSS reference model as defined by [this Plattform Industrie 4.0 Whitepaper](https://www.plattform-i40.de/IP/Redaktion/DE/Downloads/Publikation/CapabilitiesSkillsServices.html). All definitions according to this whitepaper can be found in a [Wiki page](https://github.com/hsu-aut/css-ontology/wiki/Definitions). The most important elements of this reference model are: