Update README.md

Author: Unai Abrisketa

README.md

@@ -5,6 +5,21 @@
 [![Maintainability](https://api.codeclimate.com/v1/badges/1d173fa0b393e8eaf2a2/maintainability)](https://codeclimate.com/github/ePages-de/beyond_api-ruby_client/maintainability)
 ![License](https://img.shields.io/github/license/ePages-de/beyond_api-ruby_client)
 
+## Get your credentials
+
+### Sign up for a developer test shop
+
+First you will need to sign up for a Beyond test shop.
+Visit [our sign up form](https://signup.beyondshop.cloud), fill out the form with your data, and click the **Sign up** button.
+
+### Create a custom app
+
+Log in to the cockpit of your test shop, navigate to **Apps > Custom apps** and click **Add custom app**.
+Fill out the form with the **App name**, **Application Callback URL** and **App scopes**.
+Save your app.
+
+You will then receive your `client_id` and `client_secret`.
+
 ## Installation
 
 Add this line to your application's Gemfile: