Add php.ini usage

Fixes docker-library/php#18
I am all for encouraging better practices.  Should we also change the base example to be `COPY src/ /var/www/html` as well?  Is `src/` a good example directory?  We can also add a sentence to explain the directories, like: 
 > `src/` is the directory containing all your php code and `config/` contains your `php.ini` file