add readme node to composer.json schema

res/composer-schema.json

@@ -33,6 +33,10 @@
             "description": "Homepage URL for the project.",
             "format": "uri"
         },
+        "readme": {
+            "type": "string",
+            "description": "Relative path to the readme document."
+        },
         "version": {
             "type": "string",
             "description": "Package version, see https://getcomposer.org/doc/04-schema.md#version for more info on valid schemes."