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
[en] fix original text before translating
Merge pull request #2069 from blackpuppy/zh

[en] fix original text before translating
[zh_CN] done /zh/development/routing.rst
Merge pull request #2067 from ravage84/2.x-contributing-in-pdf

Add Contributing to the PDF version of the book
Merge pull request #2071 from blackpuppy/zh

[zh_CN] done /zh/development/routing.rst
Don't include both blog tutorials in the PDF & EPUB version

https://github.com/cakephp/docs/issues/2059#issuecomment-65059586
[fr] update pdf and epub content
Bring epub and PDF chapter order in sync

At least the first three:
1. getting-started
2. installation
3. cakephp-overview
Update simple-acl-controlled-application.rst
Update cakephp-conventions.rst
Merge pull request #2075 from eBug/patch-1

Update cakephp-conventions.rst
Merge pull request #2074 from wethecode/patch-1

Update simple-acl-controlled-application.rst
Merge pull request #2073 from cakephp/2.x-epub-pdf-chapter-order

Bring epub and PDF chapter order in sync
[zh_CN] done /zh/development/sessions.rst
Merge branch 'master' into zh-work
Merge branch 'zh-work' into oschina