Add php.ini usage

Fixes docker-library/php#18
*personally*, I always use some kind of a 'wrapper' directory around files I copy, it just gives you more flexibility/control over which files are added *where*. So, yes, I think advocating better practice is good. Especially if it is easy to follow (even for newbies). Would it be a good thing to add a well-documented `php.ini.sample` to the image as a starting point?