doc: mention DCO and what to do in your commit (zalando#3114)

Signed-off-by: Sandor Szücs <sandor.szuecs@zalando.de>

CONTRIBUTING.md

@@ -13,21 +13,26 @@ If you need help, please reach out to us: [community channels](https://github.co
 
 
 ## Report a bug
+
 Reporting bugs is one of the best ways to contribute. Before creating a bug report, please check that an [issue](https://github.com/zalando/skipper/issues) reporting the same problem does not already exist. If there is an such an issue, you may add your information as a comment.
 
 To report a new bug you should open an issue that summarizes the bug and set the label to "bug".
 
 If you want to provide a fix along with your bug report: That is great! In this case please send us a pull request as described in section [Contribute Code](#contribute-code).
 
 ## Suggest a feature
+
 To request a new feature you should open an [issue](https://github.com/zalando/skipper/issues/new) and summarize the desired functionality and its use case. Set the issue label to "enhancement".
 
 ## Contribute code
-This is a rough outline of what the workflow for code contributions looks like:
+
+This is a rough outline of what the workflow for code contributions
+looks like:
+
 - Check the list of open [issues](https://github.com/zalando/skipper/issues). Either assign an existing issue to yourself, or create a new one that you would like work on and discuss your ideas and use cases.
 - Fork the repository on GitHub
 - Create a topic branch, for example feature/foo fix/bar refactor/baz, from where you want to base your work. The base is usually master.
-- Make commits of logical units.
+- Make commits of logical units and use `git commit --sign-off` to comply with [DCO](https://developercertificate.org/).
 - Write good commit messages (see below).
 - Push your changes to a topic branch in your fork of the repository.
 - Submit a pull request to [zalando/skipper](https://github.com/zalando/skipper)
@@ -40,9 +45,11 @@ This is a rough outline of what the workflow for code contributions looks like:
 Thanks for your contributions!
 
 ### Code style
+
 Skipper is formatted with [gofmt](https://golang.org/cmd/gofmt/). Please run it on your code before making a pull request. The coding style suggested by the Golang community is the preferred one for the cases that are not covered by gofmt, see the [style doc](https://github.com/golang/go/wiki/CodeReviewComments) for details.
 
 ### Commit messages
+
 Your commit messages ideally can answer two questions: what changed and why. The subject line should feature the “what” and the body of the commit should describe the “why”.
 
 When creating a pull request, its comment should reference the corresponding issue id.