MINOR: Add --help to DumpLogSegments

Author: Tom Bentley <tbentley@redhat.com>

Reviewers: Ismael Juma <ismael@juma.me.uk>

Closes #3351 from tombentley/help

core/src/main/scala/kafka/tools/DumpLogSegments.scala

@@ -66,11 +66,13 @@ object DumpLogSegments {
     val transactionLogOpt = parser.accepts("transaction-log-decoder", "if set, log data will be parsed as " +
       "transaction metadata from the __transaction_state topic.")
 
-    if(args.length == 0)
+    val helpOpt = parser.accepts("help", "Print usage information.")
-      CommandLineUtils.printUsageAndDie(parser, "Parse a log file and dump its contents to the console, useful for debugging a seemingly corrupt log segment.")
 
     val options = parser.parse(args : _*)
 
+    if(args.length == 0 || options.has(helpOpt))
+      CommandLineUtils.printUsageAndDie(parser, "Parse a log file and dump its contents to the console, useful for debugging a seemingly corrupt log segment.")
+
     CommandLineUtils.checkRequiredArgs(parser, options, filesOpt)
 
     val printDataLog = options.has(printOpt) ||