feat(docs): Add hint for CustomFields configuration in plugin docs

Co-authored-by: Kevin Mattutat <kevin.mattutat@spaceparrots.de>

docs/content/docs/plugins/writing-a-vendure-plugin.md

@@ -38,6 +38,9 @@ import { VendurePlugin } from '@vendure/core';
 export class RandomCatPlugin {}
 ```
 
+Changing the database schema by `CustomFields` configuration requires a synchronized database. See [Customizing Models With Custom Fields]({{< relref "customizing-models" >}}) for more details.
+
+
 ### Step 2: Create a service to fetch the data
 
 Now we will create a service which is responsible for making the HTTP call to the random.cat API and returning the URL of a random cat image: