Revamped the documentation about "Contributing Docs"

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | -

This is the first step into completely revamping the *Contributing* section documentation. The main changes are:

  * A much more friendly and welcoming tone.
  * Thorough and step-by-step explanations that assumes no previous knowledge from the reader.
  * The use of real examples to illustrate the explanations, instead of using abstract concepts. 
  * The old [Managing Releases](http://symfony.com/doc/current/contributing/documentation/overview.html#managing-releases) section has been completely removed. This has nothing to do with contributing documentations. It's just documentation for a process that only concerns to doc maintainers.
You are right. In your opinion, which would be the command that we should suggest them in order to work with any Git config? Maybe `$git push -u origin improve_install_chapter` ?