Enhance dco.yml with detailed comments

Added comments explaining the purpose and usage of dco.yml.

Author: colindean

.github/dco.yml

@@ -1,3 +1,15 @@
+####
+# This file configures dco2, a tool Target uses to nudge contributors to sign-off their
+# adherence to the DCO agreement.
+#
+# dco2: https://github.com/cncf/dco2
+#
+# For more information about Target's use of the Developer Certificate of Origin standard,
+# please see https://github.com/target/.github.
+#
+# This file should be kept in sync with https://github.com/target/.github/blob/main/dco.yml
+#
+####
 # https://github.com/cncf/dco2?#remediation-commits
 allowRemediationCommits:
   # Allow individual remediation commits