Add new "special" contribution kind

We agreed to use this for special contribution items like e.g. the
initial kredits balance for core contributors from before we tracked
everything.

schemas/contribution.json

@@ -47,7 +47,8 @@
         "design",
         "ops",
         "docs",
-        "community"
+        "community",
+        "special"
       ]
     },
     "description": {