Added "XML Parsing with PHP" Book

This book covers parsing and validating XML documents, leveraging XPath expressions, and working with namespaces as well as how to create and modify XML files programmatically.

README.md

 * [Securing PHP: Core Concepts](https://leanpub.com/securingphp-coreconcepts) - A book about common security terms and practices for PHP by Chris Cornutt.
 * [Modernising Legacy Applications in PHP](https://leanpub.com/mlaphp) - A book about modernising legacy PHP applications by Paul M. Jones.
 * [Modern PHP New Features and Good Practices](http://shop.oreilly.com/product/0636920033868.do) - A book about new PHP features and best practices by Josh Lockhart.
+* [XML Parsing with PHP](http://www.phparch.com/books/xml-parsing-with-php/) - This book covers parsing and validating XML documents, leveraging XPath expressions, and working with namespaces as well as how to create and modify XML files programmatically.
 
 ## Other Books
 *Books related to general computing and web development.*