[Password manager Python tests] Add a comment on logging

This CL adds a comment on how logging is used to report script and test failures.

R=battre@chromium.org
BUG=468245

Review URL: https://codereview.chromium.org/1015533007

Cr-Commit-Position: refs/heads/master@{#321346}

components/test/data/password_manager/automated_tests/run_tests.py

@@ -18,6 +18,14 @@ of the following structure:
   tests_in_parallel=<number of parallel tests>
   # |tests_to_runs| field is optional, if it is absent all tests will be run.
   tests_to_run=<test names to run, comma delimited>
+
+The script uses the Python's logging library to report the test results,
+as well as debugging information. It emits three levels of logs (in
+descending order of severity):
+  logging.INFO: Summary of the tests.
+  logging.DEBUG: Details about tests failures.
+  SCRIPT_DEBUG (see below): Debug info of this script.
+You have to set up appropriate logging handlers to have the logs appear.
 """
 import argparse
 import ConfigParser