maintainers: add pshirshov

maintainers/maintainer-list.nix

···
     githubId = 33375;
     name = "Peter Sanford";
   };
+  pshirshov = {
+    email = "[e-mail]";
+    github = "pshirshov";
+    githubId = 295225;
+    name = "Pavel Shirshov";
+  };
   psibi = {
     email = "[e-mail]";
     github = "psibi";