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

base/test/launcher/test_launcher.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/test/launcher/test_launcher.h"
 
 #include <memory>
 
 #include "base/at_exit.h"
 #include "base/bind.h"
@@ skipping 824 matching lines @@
 
   if (command_line->HasSwitch(switches::kTestLauncherFilterFile)) {
     base::FilePath filter_file_path = base::MakeAbsoluteFilePath(
         command_line->GetSwitchValuePath(switches::kTestLauncherFilterFile));
     std::string filter;
     if (!ReadFileToString(filter_file_path, &filter)) {
       LOG(ERROR) << "Failed to read the filter file.";
       return false;
     }
 
+    // Parse the file contents (see //testing/buildbot/filters/README.md
+    // for file syntax and other info).
     std::vector<std::string> filter_lines = SplitString(
         filter, "\n", base::TRIM_WHITESPACE, base::SPLIT_WANT_ALL);
     for (const std::string& filter_line : filter_lines) {
       if (filter_line.empty() || filter_line[0] == '#')
         continue;
 
       if (filter_line[0] == '-')
         negative_test_filter_.push_back(filter_line.substr(1));
       else
         positive_test_filter_.push_back(filter_line);
@@ skipping 311 matching lines @@
   }
 
   std::string snippet(full_output.substr(run_pos));
   if (end_pos != std::string::npos)
     snippet = full_output.substr(run_pos, end_pos - run_pos);
 
   return snippet;
 }
 
 }  // namespace base