Add Twilio.

README.md

 * [Twitter OAuth](https://github.com/widop/twitter-oauth) - A library to interface with Twitter's OAuth workflow.
 * [Twitter REST](https://github.com/widop/twitter-rest) - A library to interact with Twitter's REST API.
 * [Dropbox SDK](https://github.com/dropbox/dropbox-sdk-php) - The official PHP Dropbox SDK library.
+* [Twilio](https://github.com/twilio/twilio-php) - The official Twilio PHP REST API.
 
 ## Miscellaneous
 *Useful libraries or tools that don't fit in the categories above.*