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/
Yeah, you are right (I must have spaced it). I guess this is assuming knowledge of composer and how its autoloading works, which won't be the case for composer newcomers. hmm.. Could use a link to the composer autoloading docs or maybe add a line like you suggested to explain that you have to

"add the `require 'vendor/autoload.php'` somewhere."

in order for it to actually be usable. 

But yes. Could use some clarification for composer newcomers and as use as the singular dependency.