Metrics - Add actions/README.md

base/metrics/user_metrics.h

Index: base/metrics/user_metrics.h
diff --git a/base/metrics/user_metrics.h b/base/metrics/user_metrics.h
index 93701e8fd209897a144af07dcab0f197cc8c6ff9..87fbd9cac0cc81a6a4258fc58be1afcb5112fdae 100644
--- a/base/metrics/user_metrics.h
+++ b/base/metrics/user_metrics.h
@@ -17,6 +17,9 @@ namespace base {
 // This module provides some helper functions for logging actions tracked by
 // the user metrics system.
 
+// For best practices on deciding when to emit a user action, see
+// https://chromium.googlesource.com/chromium/src.git/+/HEAD/tools/metrics/actions/README.md
+
 // Record that the user performed an action.
 // This function must be called after the task runner has been set with
 // SetRecordActionTaskRunner().