Added link to the sandbox

Author: felixcheruiyot

README.md

@@ -4,6 +4,12 @@
 
 ## Official documentation
 
+PHP SDK for [IntaSend Payment Gateway](https://intasend.com). IntaSend enables you to easily add payments to your application with a few lines of code.
+
+Follow the instruction below to install and get started.
+
+Visit our [sandbox/developers](https://sandbox.intasend.com) test for your API Keys.
+
 Checkout our [API documentation](https://developers.intasend.com/) for more details and for payload references.
 
 ## How to install
@@ -204,4 +210,4 @@ Checkout API generates a URL that enables you to do M-Pesa collection and other
 1. [Developers Documentation](https://developers.intasend.com)
 2. [Test and sandbox environment](https://sandbox.intasend.com)
 3. [Support](https://support.intasend.com)
-4. [Telegram](https://t.me/joinchat/2vIT1nrYvkc0MWQ0)
+4. [Telegram](https://t.me/joinchat/2vIT1nrYvkc0MWQ0)