Update Sandbox API key procedure. (#100)

Author: sandervdo

README.md

@@ -10,7 +10,8 @@ Now you can build even bigger and better apps and integrate them with your bank
 
 Before you dive into this brand new SDK, please consider:
 - Checking out our new developer’s page [https://bunq.com/en/developer](https://bunq.com/en/developer) 🙌  
-- Grabbing your production API key from the bunq app or asking our support for a Sandbox API key 🗝
+- Grabbing your production API key from the bunq app or generate a Sandbox API key using the
+[Tinker endpoint](https://together.bunq.com/topic/tinker-the-api) 🗝
 - Visiting [together.bunq.com](https://together.bunq.com) where you can share your creations,
 questions and experience 🎤