add documentation for verify-boilerplate script

contributors/devel/sig-testing/verify-tests.md

@@ -132,3 +132,14 @@ script:
   "${KUBE_ROOT}/.kazelcfg.json" \
   "${KUBE_ROOT}/build/root/.kazelcfg.json"
 ```
+
+## `verify-boilerplate`
+
+This script checks for the boilerplate header for all the files,
+whether the header is correct or wrong. The script collects all the
+file names generated by `hack/boilerplate/boilerplate.py` script and
+stores them into a list called `files_need_boilerplate`.
+
+Once we collect all the file names, we run a check to identify the
+files with wrong header. This check will only run if the file exists
+in the list mentioned above.