Explain sign-off requirements (#417)

CONTRIBUTING.md

@@ -114,6 +114,7 @@ pull request is most likely to be accepted if it:
   review comments](https://github.com/golang/go/wiki/CodeReviewComments).
 * Has a [good commit
   message](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html).
+* Each commit must be signed by the author ([see below](#sign-your-work)).
 
 ## License
 
@@ -138,3 +139,65 @@ way to add such header is to run `make fmt`.
 // See the License for the specific language governing permissions and
 // limitations under the License.
 ```
+
+## Sign your work
+
+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 (from
+[developercertificate.org](http://developercertificate.org/)):
+
+```
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+660 York Street, Suite 102,
+San Francisco, CA 94110 USA
+
+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.
+```
+
+then you just add a line to every git commit message:
+
+    Signed-off-by: Joe Smith <[email protected]>
+
+using your real name (sorry, no pseudonyms or anonymous contributions.)
+
+You can add the sign off when creating the git commit via `git commit -s`.
+
+If you want this to be automatic you can set up some aliases:
+
+```
+git config --add alias.amend "commit -s --amend"
+git config --add alias.c "commit -s"
+```