Add indications for contributing (draft, has to be improved)

Highly inspired by: https://raw.github.com/hotosm/HDM-CartoCSS/master/CONTRIBUTING.md
Update CONTRIBUTING.md
update CONTRIBUTING: tab indent in Java
Tab indent

Conflicts:
	extractorapp/src/main/java/org/georchestra/extractorapp/ws/extractor/ExtractorController.java
remove trailing spaces

Conflicts:
	extractorapp/src/main/java/org/georchestra/extractorapp/ws/extractor/ExtractorController.java
align spaces