README.md: State DCO requirement explicitly

Over the last commits and Pull Requests, I have started to enforce the
use of the Developer's Certificate of Origin (DCO) through signed-off
commits for this project. Let's make the requirement explicitly stated
in the README.md file.

For the record, the DCO I refer to as I introduce this requirement is
available at https://developercertificate.org/, and reads as follows:

    Developer Certificate of Origin
    Version 1.1

    Copyright (C) 2004, 2006 The Linux Foundation and its contributors.

    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.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>

README.md

@@ -558,6 +558,10 @@ in some places instead of the Rusty look the author would like it to have. So
 feedback (or PRs) are welcome, including about ways you might see to take
 better advantage of Rust features.
 
+Note that the project expects new commits to be covered by the
+[Developer's Certificate of Origin](https://wiki.linuxfoundation.org/dco).
+When contributing Pull Requests, please sign off your commits accordingly.
+
 ## Questions / Answers
 
 ### Why implementing an eBPF virtual machine in Rust?