Update list of contributors for upcoming v1.21.0

Author: stephane-caron

CITATION.cff

@@ -42,3 +42,5 @@ authors:
 - family-names: "Jallet"
   given-names: "Wilson"
   orcid: "https://orcid.org/0000-0001-8222-2739"
+- family-names: "Zhang"
+  given-names: "Yutong"

README.md

@@ -331,10 +331,10 @@ If you use this project in your works, please cite as follows:
 ```bibtex
 @software{robot_descriptions_py,
   title = {{robot_descriptions.py: Robot descriptions in Python}},
-  author = {Caron, Stéphane and Romualdi, Giulio and Kozlov, Lev and Ordoñez Apraez, Daniel Felipe and Tadashi Kussaba, Hugo and Bang, Seung Hyeon and Zakka, Kevin and Schramm, Fabian and Uru\c{c}, Jafar and Traversaro, Silvio and Castro, Sebastian and Tao, Haixuan Xavier and Jallet, Wilson},
+  author = {Caron, Stéphane and Romualdi, Giulio and Kozlov, Lev and Ordoñez Apraez, Daniel Felipe and Tadashi Kussaba, Hugo and Bang, Seung Hyeon and Zakka, Kevin and Schramm, Fabian and Uru\c{c}, Jafar and Traversaro, Silvio and Castro, Sebastian and Tao, Haixuan Xavier and Jallet, Wilson and Zhang, Yutong},
   license = {Apache-2.0},
   url = {https://github.com/robot-descriptions/robot_descriptions.py},
-  version = {1.20.0},
+  version = {1.21.0},
   year = {2025}
 }
 ```