add doco for installing with composer

Add doco in the readme for installing with composer.

You probably don't want to take it this far, but I reckon the composer instructions should not only be added, but also be pushed higher up and marked as the recommended way to install. The quicker everyone in the PHP community (including newbies) takes on composer/packagist as the defacto way to handle dependencies, the better.

Signed-off-by: Joshua Paling <joshua.paling@gmail.com>