Add Contributing to the PDF version of the book

The 3.x version of the book also has the Contributing section:
https://github.com/cakephp/docs/blame/66cc98a8775ceb67f627d4f3f54f1b3215e51f0e/en/pdf-contents.rst#L11
Merge pull request #2067 from ravage84/2.x-contributing-in-pdf

Add Contributing to the PDF version of the book