CONTRIBUTING: add DCO

Signed-off-by: Salvatore Mesoraca <[email protected]>

CONTRIBUTING.md

@@ -14,6 +14,62 @@ If you don't know where to start, consider improving
 documentation, bug triaging, and writing tutorials are all examples of
 helpful contributions that mean less work for you.
 
+## Developer Certificate of Origin
+
+We respect the intellectual property rights of others and we want to make sure
+all incoming contributions are correctly attributed and licensed. A Developer
+Certificate of Origin (DCO) is a lightweight mechanism to do that. The DCO is
+a declaration attached to every commit. In the commit message of the contribution,
+the developer simply adds a `Signed-off-by` statement and thereby agrees to the DCO,
+which you can find below or at [DeveloperCertificate.org](http://developercertificate.org/).
+
+```text
+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.
+```
+
+We require that every contribution to Valkey to be signed with a DCO. We require the
+usage of known identity (such as a real or preferred name). We do not accept anonymous
+contributors nor those utilizing pseudonyms. A DCO signed commit will contain a line like:
+
+
+```text
+Signed-off-by: Jane Smith <[email protected]>
+```
+
+You may type this line on your own when writing your commit messages. However, if your
+user.name and user.email are set in your git configs, you can use `git commit` with `-s`
+or `--signoff` to add the `Signed-off-by` line to the end of the commit message. We also
+require revert commits to include a DCO.
+
+If you're contributing code to the Valkey project in any other form, including
+sending a code fragment or patch via private email or public discussion groups,
+you need to ensure that the contribution is in accordance with the DCO.
+
 ## Your First Contribution
 
 Unsure where to begin contributing? You can start by looking through