Add Link to Awesome-AppSec PHP

As suggested by sarciszewski on reddit/r/php: 
- https://www.reddit.com/r/PHP/comments/a6w4hy/awesome_php_security_security_related_resources/ebynuvl/

Author: streichsbaer

README.md

@@ -55,6 +55,7 @@ Supported by: [GuardRails.io](https://github.com/apps/guardrails)
 - [Official PHP Security Manual](http://php.net/manual/en/security.php)
 - [Survive The Deep End: PHP Security](https://phpsecurity.readthedocs.io/en/latest/)
 - [Security Tips for a PHP Application](https://dev.to/restoreddev/security-tips-for-a-php-application-4e9a)
+- [Awesome-AppSec: PHP-Section](https://github.com/paragonie/awesome-appsec#php)
 
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