GH-1089 - Set up DCO infrastructure.

Author: odrotbohm

Diff for: .github/dco.yml

@@ -0,0 +1,2 @@
+require:
+  members: false

Diff for: CONTRIBUTING.adoc

@@ -0,0 +1,5 @@
+= Contributing to Spring Modulith
+
+* As a contributor to this project you aggree to follow the Spring Project https://github.com/spring-projects/.github/blob/main/CODE_OF_CONDUCT.md[code of conduct].
+* 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].