Add simple explanation to verify-spelling.sh

hack/verify-spelling.sh

@@ -13,6 +13,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# This script checks commonly misspelled English words in all files in the
+# working directory by client9/misspell package.
+# Usage: hack/verify-spelling.sh
+
 set -o errexit
 set -o nounset
 set -o pipefail