Add beberlei/assert to Filtering and Validation

README.md

 * [Upload](https://github.com/brandonsavage/Upload) - A library for handling file uploads and validation.
 * [Valitron](https://github.com/vlucas/valitron) - Another validation library.
 * [Volan](https://github.com/serkin/Volan) - Another simplified validation library.
+* [Assert](https://github.com/beberlei/assert) - A validation library with a rich set of assertions. Supports assertion chaining and lazy assertions.
 
 ## API
 *Libraries and web tools for developing APIs.*