contributing.md: Add developer certificate of origin stuff

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

docs/contributions.md

@@ -32,3 +32,43 @@ code sample in doxygen.  Everything in between such tags will show up in the
 class documentation when the following is put at the end of a class docstring
 as show above.
 
+Code signing
+============
+
+The sign-off is a simple line at the end of the explanation for the
+patch, which certifies that you wrote it or otherwise have the right to pass it
+on as an open-source patch.  The rules are pretty simple: if you can certify
+the below:
+
+        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.
+
+then you just add a line saying
+
+        Signed-off-by: Random J Developer <random@developer.example.org>
+
+using your real name (sorry, no pseudonyms or anonymous contributions.)
+Unsigned commits will not be accepted.