Document test_zeroize.gdb script
diff --git a/tests/scripts/test_zeroize.gdb b/tests/scripts/test_zeroize.gdb
index 52b7cda..15b8b09 100644
--- a/tests/scripts/test_zeroize.gdb
+++ b/tests/scripts/test_zeroize.gdb
@@ -1,3 +1,20 @@
+# test_zeroize.gdb
+#
+# This file is part of mbed TLS (https://tls.mbed.org)
+#
+# Copyright (c) 2017, ARM Limited, All Rights Reserved
+#
+# Purpose
+#
+# Run a test using the debugger to check that the mbedtls_zeroize() function in
+# utils.h is not being optimized out by the compiler. To do so, the script
+# loads the test program at programs/test/zeroize.c and sets a breakpoint at
+# the last return statement in the main(). When the breakpoint is hit, the
+# debugger manually checks the contents to be zeroized and checks that it is
+# actually cleared.
+#
+# Note: This test requires that the test program is compiled with -g3.
+
set confirm off
file ./programs/test/zeroize
break zeroize.c:90
