Add avatar for jclsn

Author: jclsn

content/images/avatars/jclsn.jpg

@@ -285,6 +285,10 @@ def __init__(self, url, title, context, css="", children=()):
         "email": "mixxx@acolombier.dev",
         "tagline": "Mixxx Core Developer",
     },
+    "Jan Claußen": {
+        "github": "jclsn",
+        "tagline": "Mixxx Contributor",
+    },
 }
 
 # Needed for Jinja2 markdown filter