Update readme

Author: drmonkeyninja

Diff for: readme.md

@@ -1,5 +1,7 @@
 # Example PayPal REST API Integration with PHP and MySQL
 
+The code in this repository is from our blog post [Using PayPal's REST API with PHP and MySQL](https://www.evoluted.net/thinktank/web-development/using-paypals-rest-api-with-php-and-mysql).
+
 * src/index.html - contains the form for making a payment
 * src/bootstrap.php - contains the configuration for the request/response
 * src/request.php - contains the request code for the payment