Add note indicating modes have usage text (#794)

cmd/nebula-cert/main.go

@@ -127,6 +127,8 @@ func help(err string, out io.Writer) {
 	fmt.Fprintln(out, "    "+signSummary())
 	fmt.Fprintln(out, "    "+printSummary())
 	fmt.Fprintln(out, "    "+verifySummary())
+	fmt.Fprintln(out, "")
+	fmt.Fprintf(out, "  To see usage for a given mode, use %s <mode> -h\n", os.Args[0])
 }
 
 func mustFlagString(name string, val *string) error {

cmd/nebula-cert/main_test.go

@@ -22,7 +22,9 @@ func Test_help(t *testing.T) {
 		"    " + keygenSummary() + "\n" +
 		"    " + signSummary() + "\n" +
 		"    " + printSummary() + "\n" +
-		"    " + verifySummary() + "\n"
+		"    " + verifySummary() + "\n" +
+		"\n" +
+		"  To see usage for a given mode, use " + os.Args[0] + " <mode> -h\n"
 
 	ob := &bytes.Buffer{}