add credits to changelog

CHANGELOG.md

@@ -7,6 +7,7 @@ API-breaking change is listed.
   and `{[ (x,y,z) ]}`. They date back to the time we used the `singleton` class
   with a product for maps (there's now the `singletonM` class).
 - Add map notations `{[ k1 := x1 ; .. ; kn := xn ]}` up to arity 13.
+  (by Lennard Gäher)
 - Add multiset literal notation `{[+ x1; .. ; xn +]}`.
   + Add a new type class `SingletonMS` (with projection `{[+ x +]}` for
     multiset singletons.