Add standard logo/link to sign up

Author: richleland

README.md

@@ -1,7 +1,12 @@
+<a href="https://www.sparkpost.com"><img src="https://www.sparkpost.com/sites/default/files/attachments/SparkPost_Logo_2-Color_Gray-Orange_RGB.svg" width="200px"/></a>
+
+[Sign up](https://app.sparkpost.com/sign-up?src=Dev-Website&sfdcid=70160000000pqBb) for a SparkPost account and visit our [Developer Hub](https://developers.sparkpost.com) for even more content.
+
+# SparkPost PHP SDK
+
 [![Travis CI](https://travis-ci.org/SparkPost/php-sparkpost.svg?branch=master)](https://travis-ci.org/SparkPost/php-sparkpost)
 [![Coverage Status](https://coveralls.io/repos/SparkPost/php-sparkpost/badge.svg?branch=master&service=github)](https://coveralls.io/github/SparkPost/php-sparkpost?branch=master)
 
-# SparkPost PHP SDK
 The official PHP binding for your favorite SparkPost APIs!
 
 Before using this library, you must have a valid API Key.