composer.json addition and README.md formatting fix

Hi there!

I am creating a pull request as per our conversation on the ARVE community discussion forums (bit.ly/1uHoYS5)

I authored a composer.json file following the formatting/guidelines of the following resources:
* Yoast's WordPress SEO Plugin composer.json - https://raw.githubusercontent.com/Yoast/wordpress-seo/master/composer.json
* roots.io "WordPress Plugins with Composer" - http://roots.io/wordpress-plugins-with-composer/
* Official Composer Schema doc - https://getcomposer.org/doc/04-schema.md

Please review the information within the composer.json as it contains author and project details that you should approve/be aware of. I used the information already published on/within the plugin to fill everything out that I could.

I also fixed some basic newline formatting within your README.md file (was using for the composer.json update) because really important details (like your donate link, etc.) are shrink-wrapping themselves onto one line.

Thanks again for such a great plugin!
- John