Added the PHP package checklist to the PHP reading section. (#625)

README.md

@@ -914,6 +914,7 @@ Various resources, such as books, websites and articles, for improving your PHP
 * [Don't Worry About BREACH](http://blog.ircmaxell.com/2013/08/dont-worry-about-breach.html) - An article about the BREACH hack and CSRF tokens.
 * [On PHP 5.3, Lambda Functions and Closures](http://fabien.potencier.org/on-php-5-3-lambda-functions-and-closures.html) - An article about lambda functions and closures.
 * [PHP Is Much Better Than You Think](http://fabien.potencier.org/php-is-much-better-than-you-think.html) - An article about the PHP language and ecosystem.
+* [PHP Package Checklist](http://phppackagechecklist.com/) - A checklist for successful PHP package development.
 * [PHP Sucks! But I Like It!](http://blog.ircmaxell.com/2012/04/php-sucks-but-i-like-it.html) - An article about the pros and cons of PHP.
 * [Preventing CSRF Attacks](http://blog.ircmaxell.com/2013/02/preventing-csrf-attacks.html) - An article on preventing CSRF attacks.
 * [Seven Ways to Screw Up BCrypt](http://blog.ircmaxell.com/2012/12/seven-ways-to-screw-up-bcrypt.html) - An article about correct BCrypt implementation.