Updated Zend Framework Git Guide url

Possible fix for #1338

Perhaps you want to fix the error on this url in CONTRIBUTING.md: [Zend Framework Git Guide](http://framework.zend.com/wiki/display/ZFDEV2/Zend+Framework+Git+Guide), but linking to https://github.com/zendframework/zf2/blob/master/README-GIT.md might be better?