Add new config option to json schema

res/composer-schema.json

@@ -175,6 +175,10 @@
                 "discard-changes": {
                     "type": ["string", "boolean"],
                     "description": "The default style of handling dirty updates, defaults to false and can be any of true, false or \"stash\"."
+                },
+                "prepend-autoloader": {
+                    "type": "boolean",
+                    "description": "If false, the composer autoloader will not be prepended to existing autoloaders, defaults to true."
                 }
             }
         },