Add express as peer dependency (#907)

Author: marvinruder

package.json

@@ -70,5 +70,8 @@
     "supertest": "^6.2.2",
     "ts-node": "^10.7.0",
     "typescript": "^4.6.2"
+  },
+  "peerDependencies": {
+    "express": "*"
   }
 }