Added editorconfig

| Q   | A
| --- | ---
| Doc fix? | no
| New docs? | no
| Applies to | all
| Fixed tickets | -

[EditorConfig](http://editorconfig.org/) is a very great project to maintain a
coding styles. It uses editor plugins for a dozen editors. This way, a file is
automatically fixed when the file is opened/written.

I think it'll be pretty usefull to use in the docs.

(fyi, symfony code has this for a long time already)