Add a helpful usage if given no arguments

bin/perl_sec_audit

@@ -6,6 +6,9 @@ my @files = @ARGV;
 my $params = {
     -theme => 'security',
 };
+
+usage() unless @files;
+
 for my $file (@files) {
     next unless -f $file;
     my @critique = critique($params, $file);
@@ -15,3 +18,7 @@ for my $file (@files) {
         print "\n";
     }
 }
+
+sub usage {
+    say "$0 file1 file2 ...";
+}