Update README.md

Author: Amine Mouafik

README.md

@@ -8,5 +8,15 @@
 
 ## Usage
 
+```javascript
+var AuhorizeNet = require('authorize-net');
+var client = new AuthorizeNet({
+  API_LOGIN_ID: '<PLACEHOLDER>',
+  TRANSACTION_KEY: '<PLACEHOLDER>'
+});
+```
+
+## Gateway API
+
 This SDK is natively compatible with [42-cent](https://github.com/continuous-software/42-cent).  
 It implements the [BaseGateway](https://github.com/continuous-software/42-cent-base) API.