chore: add Volker Diels-Grabsch as contributor

Signed-off-by: Yukai Chou <[email protected]>

Author: muzimuzhi

doc/generic/pgf/CHANGELOG.md

@@ -40,6 +40,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Yukai Chou (@muzimuzhi)
 - Alexander Grahn
 - Max Chernoff
+- Volker Diels-Grabsch
 
 ## [3.1.10] - 2023-01-13 Henri Menke