doc: State the DCO sign-off as required, not optional

The opening bullet said contributions must comply with the DCO while the
pull-request section called the sign-off optional two paragraphs later.
Decision: required — see brain decision 0008.

Signed-off-by: Simon Walz <contact@simonwalz.de>

Author: simonwalz

CONTRIBUTING.md

@@ -51,8 +51,8 @@ If you encounter a bug or have a feature request:
   2 Make your changes and commit them:
     `git commit -m "feat: your feature description"`
 
-Note: You can signed-off your commits by `git commit -s ...`.
-See [Developer Certificate of Origin (DCO)](DCO.txt).
+  3 Sign off your commits with `git commit -s`. This is required for every
+    commit — see [Developer Certificate of Origin (DCO)](DCO.txt).
 
 ## 🤝 Community Guidelines