doc: add dco to github pr template

Our project relies on the Developer Certificate of Origin as a passive
alternative to a Contributor License Agreement. The PR template seems
like a great place to surface the text to help new contributors be
aware of the DCO and it's implications.

PR-URL: #24023
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -14,3 +14,31 @@ Contributors guide: https://github.com/nodejs/node/blob/master/CONTRIBUTING.md
 - [ ] tests and/or benchmarks are included
 - [ ] documentation is changed or added
 - [ ] commit message follows [commit guidelines](https://github.com/nodejs/node/blob/master/doc/guides/contributing/pull-requests.md#commit-message-guidelines)
+
+<!--
+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.
+-->