maintainers: add icewind1991

maintainers/maintainer-list.nix

···
     githubId = 3661115;
     name = "Ingo Blechschmidt";
   };
+  icewind1991 = {
+    name = "Robin Appelman";
+    email = "[e-mail]";
+    github = "icewind1991";
+    githubId = 1283854;
+  };
   icy-thought = {
     name = "Icy-Thought";
     email = "[e-mail]";