Added Bugsnag to Debugging / Profiling

README.md

 * [APM](http://pecl.php.net/package/APM) - Monitoring extension collecting errors and statistics into SQLite/MySQL/StatsD.
 * [Barbushin PHP Console](https://github.com/barbushin/php-console) - Another web debugging console using Google Chrome.
 * [Blackfire.io](https://blackfire.io) - A low-overhead code profiler.
+* [BugSnag](https://www.bugsnag.com/platforms/php/) - Logs bugs. Includes useful debugging info like session, release, etc. Has a free tier / is free for open source. 
 * [Kint](https://github.com/kint-php/kint) - A debugging and profiling tool.
 * [PHP Console](https://github.com/Seldaek/php-console) - A web debugging console.
 * [PHP Debug Bar](http://phpdebugbar.com/) - A debugging toolbar.