Add Dompdf for working with PDF

A PHP-only alternative to WKHTMLToPDF.

README.md

 * [Snappy](https://github.com/KnpLabs/snappy) - A PDF and image generation library.
 * [WKHTMLToPDF](https://github.com/wkhtmltopdf/wkhtmltopdf) - A tool to convert HTML to PDF.
 * [PHPPdf](https://github.com/psliwa/PHPPdf) - A library for generating PDFs and images from XML.
+* [Dompdf](https://github.com/dompdf/dompdf) - A HTML to PDF converter.
 
 ## Office
 *Libraries for working with office suite documents.*