CONTRIBUTING.md: Describe how to signoff on the commit

This adds description of the Developer Certificate of Origin

Signed-off-by: Stef Walter <[email protected]>

CONTRIBUTING.md

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+# Developer Certificate of Origin
+
+Add a ```Signed-off-by``` line to commit messages to indicate that you
+certify that you wrote or otherwise hav ethe right to submit the code that
+you are contributing to the project.
+
+Here is the [full text of the DCO](https://developercertificate.org/)
+
+Contributors sign-off that they adhere to these requirements by adding a
+```Signed-off-by``` line to commit messages.
+
+This is my commit message
+
+    Signed-off-by: Random J Developer <[email protected]>
+
+Git even has a -s command line option to append this automatically to your commit message:
+
+    $ git commit -s -m 'This is my commit message'
+
+