Add PHP 7 Upgrade Guide e-book

README.md

 ## PHP Books
 *Fantastic PHP-related books.*
 
+* [PHP 7 Upgrade Guide](https://leanpub.com/php7) - An ebook covering all of the features and changes in PHP 7 by Colin O'Dell.
 * [PHP Pandas](http://daylerees.com/php-pandas/) - A book about learning to write PHP by Dayle Rees.
 * [Scaling PHP Applications](http://www.scalingphpbook.com) - An ebook about scaling PHP applications by Steve Corona.
 * [The Grumpy Programmer's Guide to Building Testable PHP Applications](https://leanpub.com/grumpy-testing) - A book about building testing PHP applications by Chris Hartjes.