Document test data makefile
diff --git a/tests/data_files/print_c.pl b/tests/data_files/print_c.pl
index b584578..d0ec137 100755
--- a/tests/data_files/print_c.pl
+++ b/tests/data_files/print_c.pl
@@ -1,10 +1,19 @@
#!/usr/bin/env perl
-# Usage:
-# print_c mbedtls_test_foo <file.pem
-# print_c TEST_FOO mbedtls_test_foo <file.pem
use strict;
use warnings;
+if (!@ARGV || $ARGV[0] == '--help') {
+ print <<EOF;
+Usage: $0 mbedtls_test_foo <file.pem
+ $0 TEST_FOO mbedtls_test_foo <file.pem
+Print out a PEM file as C code defining a string constant.
+
+Used to include some of the test data in /library/certs.c for
+self-tests and sample programs.
+EOF
+ exit;
+}
+
my $pp_name = @ARGV > 1 ? shift @ARGV : undef;
my $name = shift @ARGV;