Updates README. Adds Composer Install Instructions

This package is on Packagist (https://packagist.org/packages/michelf/php-markdown). 
This package promotes PSR autoloading. 
Hence, this package should be installed via Composer and promote the autoloading capabilities provided by Composer.

Questions about Composer? See: http://blog.doh.ms/2014/10/13/installing-composer-packages/
Could add a line like this one: "If you are using Composer, it setups autoloading for you, see below.". Then add a "Composer" section just above the "Usage without an autoloader" section. I'd also put the installation instructions there so that everything about composer is at the same place.