Add AsyncAWS unofficial AWS SDK.

README.md

 *Libraries for accessing third party APIs.*
 
 * [Amazon Web Service SDK](https://github.com/aws/aws-sdk-php) - The official PHP AWS SDK library.
+* [AsyncAWS](https://async-aws.com/) - An unofficial asynchronous PHP AWS SDK.
 * [Campaign Monitor](https://campaignmonitor.github.io/createsend-php/) - The official Campaign Monitor PHP library.
 * [Dropbox SDK](https://github.com/dropbox/dropbox-sdk-php) - The official PHP Dropbox SDK library.
 * [Github](https://github.com/KnpLabs/php-github-api) - A library to interface with the Github API.