Add usage in some hack/update scripts

hack/update-generated-runtime.sh

@@ -14,6 +14,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# This script builds protoc-gen-gogo binary in runtime and genertates 
+# `*/api.pb.go` from the protobuf file `*/api.proto`.
+# Usage: 
+#     hack/update-generated-runtime.sh
+
 set -o errexit
 set -o nounset
 set -o pipefail