Add APIGen.

README.md

 *Libraries for generating project documentation.*
 
 * [Sami](https://github.com/fabpot/Sami) - An API documentation generator.
-* [PHP Documentor 2](https://github.com/phpDocumentor/phpDocumentor2) - Another documentation generator.
+* [APIGen](https://github.com/apigen/apigen) - Another API documentation generator.
+* [PHP Documentor 2](https://github.com/phpDocumentor/phpDocumentor2) - A documentation generator.
 
 ## Security
 *Libraries for generating secure random numbers, encrypting data and scanning for vulnerabilities.*