Add DCO text to CONTRIBUTING.md (#1885)

Signed-off-by: Michael Nelson <minelson@vmware.com>

CONTRIBUTING.md

@@ -11,6 +11,64 @@ Please note we have a code of conduct, please follow it in all your interactions
 3. Increase the version numbers in any examples files and the README.md to the new version that this Pull Request would represent. The versioning scheme we use is [SemVer](http://semver.org/).
 4. You may merge the Pull Request in once you have the sign-off of two other developers, or if you do not have permission to do that, you may request the second reviewer to merge it for you.
 
+## DCO Sign off
+
+All authors to the project retain copyright to their work. However, to ensure
+that they are only submitting work that they have rights to, we are requiring
+everyone to acknowldge this by signing their work.
+
+Any copyright notices in this repo should specify the authors as "the contributors".
+
+To sign your work, just add a line like this at the end of your commit message:
+
+```text
+Signed-off-by: Michael Nelson <minelson@vmware.com>
+```
+
+This can easily be done with the `--signoff` option to `git commit`.
+
+By doing this you state that you can certify the following (from [https://developercertificate.org/](https://developercertificate.org/):
+
+```text
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+1 Letterman Drive
+Suite D4700
+San Francisco, CA, 94129
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.
+```
+
 ## Code of Conduct
 
 ### Our Pledge