Documentation update for #3753

docs/index.md

@@ -1628,6 +1628,7 @@ In addition to supporting the legacy [`mocha.opts`](#mochaopts) run-control form
 - **JavaScript**: Create a `.mocharc.js` in your project's root directory, and export an object (`module.exports = {/* ... */}`) containing your configuration.
 - **YAML**: Create a `.mocharc.yaml` (or `.mocharc.yml`) in your project's root directory.
 - **JSON**: Create a `.mocharc.json` in your project's root directory. Comments — while not valid JSON — are allowed in this file, and will be ignored by Mocha.
+- **[JSON5](https://json5.org/)**: Create a `.mocharc.jsonc` (or `.mocharc.json5`) in your project's root directory.
 - **`package.json`**: Create a `mocha` property in your project's `package.json`.
 
 Mocha suggests using one of the above strategies for configuration instead of the legacy `mocha.opts` format.