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

base/test/launcher/test_launcher.cc

Index: base/test/launcher/test_launcher.cc
diff --git a/base/test/launcher/test_launcher.cc b/base/test/launcher/test_launcher.cc
index 364cf113c6a6eed776a3059cbbf15d307fda4c33..8528b1d5fd906be59fcb11b9648981ff6996eff2 100644
--- a/base/test/launcher/test_launcher.cc
+++ b/base/test/launcher/test_launcher.cc
@@ -842,6 +842,8 @@ bool TestLauncher::Init() {
       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) {