docs: document Node.js' --enable-source-maps flag

docs/index.md

@@ -39,6 +39,7 @@ Mocha is a feature-rich JavaScript test framework running on [Node.js][] and in
 - clickable suite titles to filter test execution
 - [node debugger support](#-inspect-inspect-brk-inspect)
 - [node native ES modules support](#nodejs-native-esm-support)
+- [source-map support](#-enable-source-maps)
 - [detects multiple calls to `done()`](#detects-multiple-calls-to-done)
 - [use any assertion library you want](#assertions)
 - [extensible reporting, bundled with 9+ reporters](#reporters)
@@ -1114,6 +1115,19 @@ These flags vary depending on your version of Node.js.
 
 `node` flags can be defined in Mocha's [configuration](#configuring-mocha-nodejs).
 
+### `--enable-source-maps`
+
+> _New in Node.js v12.12.0_
+
+If the [`--enable-source-maps`](https://nodejs.org/dist/latest-v12.x/docs/api/cli.html#cli_enable_source_maps) flag
+is passed to mocha, source maps will be collected and used to provide accurate stack traces for transpiled code:
+
+```bash
+Error: cool
+    at Object.<anonymous> (/Users/fake-user/bigco/nodejs-tasks/build/src/index.js:27:7)
+        -> /Users/fake-user/bigco/nodejs-tasks/src/index.ts:24:7
+```
+
 ### About V8 Flags
 
 Prepend `--v8-` to any flag listed in the output of `node --v8-options` (excluding `--v8-options` itself) to use it.