Gathering information about test filter files into a README.md file.

base/test/launcher/unit_test_launcher.cc

Index: base/test/launcher/unit_test_launcher.cc
diff --git a/base/test/launcher/unit_test_launcher.cc b/base/test/launcher/unit_test_launcher.cc
index b97d09f65df8f37cea66683ad557ebf26bb58be1..c9f0eb0afb139f02094dcb62408f747379fdda67 100644
--- a/base/test/launcher/unit_test_launcher.cc
+++ b/base/test/launcher/unit_test_launcher.cc
@@ -73,6 +73,7 @@ void PrintUsage() {
           "  --test-launcher-filter-file=PATH\n"
           "    Like --gtest_filter, but read the test filter from PATH.\n"
           "    One pattern per line; lines starting with '-' are exclusions.\n"
+          "    See also //testing/buildbot/filters/README.md file.\n"
           "\n"
           "  --test-launcher-batch-limit=N\n"
           "    Sets the limit of test batch to run in a single process to N.\n"