Add DCO

Author: rwinch

CONTRIBUTING.adoc

@@ -33,7 +33,7 @@ Switch to a branch named `<major>.<minor>.x` from the smallest milestone in the
 The spring team will ensure the code gets merged forward into additional branches.
 
 
-== Sign the Contributor License Agreement
-If you have not previously done so, please fill out and
-submit the https://cla.pivotal.io/sign/spring[Contributor License Agreement].
+== Add Signed-off-by Trailer
+All commits must include a __Signed-off-by__ trailer at the end of each commit message to indicate that the contributor agrees to the Developer Certificate of Origin.
+For additional details, please refer to the blog post https://spring.io/blog/2025/01/06/hello-dco-goodbye-cla-simplifying-contributions-to-spring[Hello DCO, Goodbye CLA: Simplifying Contributions to Spring].