Added documentation about the DebugFormatter helper
Added notes specifying the propagation behaviour for kernel.view and kernel.exception.
Add missing info about kernel.request event.
Changed phrasing to explain the effects off propagation stopping.
Fix incorrect event name.
Updated installation instructions to use the new Symfony Installer
Removing some installation instructions
update contribution guide for 2.7/3.0
[ExpressionLanguage] add missing argument
Restored the original section title
fixed StringExpressionLanguageProvider code example #4636
bug #4637 fixed StringExpressionLanguageProvider code example #4636 (danieleorler)

This PR was squashed before being merged into the 2.6 branch (closes #4637).

Discussion
----------

fixed StringExpressionLanguageProvider code example #4636

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.6
| Fixed tickets | #4636

Commits
-------

33e0a7f fixed StringExpressionLanguageProvider code example #4636
Minor grammar fix
Fixed section headers
Applied comments from our great reviewers
Reordered list of helpers
feature #4485 Added documentation about the DebugFormatter helper (WouterJ)

This PR was merged into the 2.6 branch.

Discussion
----------

Added documentation about the DebugFormatter helper

| Q   | A
| --- | ---
| Doc fix? | no
| New docs? | yes (symfony/symfony#10627)
| Applies to | 2.6+
| Fixed tickets | #4256

When may want to add screenshots to the article, but to get this ready quick before 2.6 is released stable, I left them out now.

Commits
-------

99f31fc Reordered list of helpers
6a02f68 Applied comments from our great reviewers
363e38f Added documentation about the DebugFormatter helper
Matching up the index position with the map
Fixed some code indentation
Fix indentation of YAML example