Add php.github.io/php-src to CONTRIBUTING.md Technical Resources list (phpGH-20580)

Author: sad270

CONTRIBUTING.md

@@ -100,6 +100,7 @@ scattered across different websites, and often outdated. Nonetheless, they can
 provide a good starting point for learning about the fundamentals of the code
 base.
 
+* https://php.github.io/php-src/
 * https://www.phpinternalsbook.com/
 * https://www.npopov.com/
   * [Internal value representation](https://www.npopov.com/2015/05/05/Internal-value-representation-in-PHP-7-part-1.html), [part 2](https://www.npopov.com/2015/06/19/Internal-value-representation-in-PHP-7-part-2.html)