docs: Add note about 'NODE_ENV=production'

Motivation graphql#2428

README.md

@@ -102,6 +102,8 @@ graphql(schema, query).then(result => {
 });
 ```
 
+**Note**: Please don't forget to set `NODE_ENV=production` if you are running a production server it will disable some checks that can be useful during development but will significantly improve performance.
+
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