Added missing library Restler.

README.md

 * [HAL](https://github.com/blongden/hal) - A Hypertext Application Language (HAL) builder library.
 * [Negotiation](https://github.com/willdurand/Negotiation) - A content negotiation library.
 * [Drest](https://github.com/leedavis81/drest) - A library for exposing Doctrine entities as REST resource endpoints.
+* [Restler](https://github.com/Luracast/Restler) - A lightweight framework to expose PHP methods as RESTful web API.
 
 ## Caching
 *Libraries for caching data.*