Update CONTRIBUTING.md

* Reflect the recent changes in @necolas's Issue Guidelines:
  https://github.com/necolas/issue-guidelines.
* Add note about not opening issues and pull requests regarding the code
  in the Apache Server Configs, jQuery, Modernizr and Normalize.css.
Update to Apache Server Configs v1.1.0
Compress .png files losslessly.

Use zopfli-png, pngout /r and deflopt.