Add missing piece to the PR template

Been awhile since I did a PR on this repo
so did not remember how the commits needed to be signed. The PR template was not clear how to get to the needed details, so adding them here.

Signed-off-by: Michael Sorens <msorens@styra.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -25,6 +25,8 @@ For more information on contributing to OPA see:
 
 * [Contributing Guide](https://www.openpolicyagent.org/docs/latest/contributing/)
   for high-level contributing guidelines and development setup.
+  (See the [Developer Certificate of Origin](https://www.openpolicyagent.org/docs/latest/contrib-code/#developer-certificate-of-origin)
+  section for specifics on signing off a commit.)
 
 -->