docs: Add DCO (expressjs#6048)

Co-authored-by: Chris de Almeida <[email protected]>

Author: 2 people

Contributing.md

@@ -207,3 +207,31 @@ dissent.  When the PR is merged, a TC member will add them to the proper GitHub/
 ### Current Initiative Captains
 
 - Triage team [ref](https://github.com/expressjs/discussions/issues/227): @UlisesGascon
+
+## Developer's Certificate of Origin 1.1
+
+```text
+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.
+```