Add php.ini usage

Fixes docker-library/php#18
I *do* like the other  example. Not sure if `src` is the *best* name, but cannot think of another name (`app`? `public_html`?) . I don't think there's a real *standard* on this in the php community, and there's *really* a wide range of people using PHP, ranging from "hey, I copied it from stackoverflow" to people that *know* what they're doing :smile: 

My guess is it's best to target the former group, because they probably need some guidance. Your note on the `src` directory looks good to me.