fix #1366 contributing.md

fix #1366 new contributing.md and codestyle.md files.
@irbrad and @sendhil - can you both review this one?

`./gradlew checkstyle` currently generate an html reports with more than 5000 style errors so it's not super useful in a pre commit hook, we also have a script that *tries* to check new style errors introduced by the current patch. I didn't want to document it because it doesn't work perfectly (it's based on a plain text diff).

IMO, we should have an indicator in the pull request with the number of new style error introduced. We could probably do that with Jenkins or Travis-ci.