Update README

Author: ehmicky

README.md

@@ -48,6 +48,14 @@ lead to conflicts between libraries. Libraries should use the
 [`-r log-process-errors` CLI flag](#usage-preferred) instead of
 [`require()`](#usage-custom).
 
+# Duplicate warnings
+
+It is recommended to use the
+[`--no-warnings` CLI flag](https://nodejs.org/api/cli.html#cli_no_warnings) or
+the
+[`NODE_NO_WARNINGS=1`](https://nodejs.org/api/cli.html#cli_node_no_warnings_1)
+environment variable to prevent warnings being logged twice.
+
 # Custom logging
 
 By default events will be logged to the console (e.g. `console.error()`).