Support index.js module resolution at startup of Electron.

docs/tutorial/quick-start.md

@@ -67,6 +67,9 @@ like this:
 }
 ```
 
+__Note__: If the `main` field is not present in `package.json`, Electron will
+attempt to load an `index.js`.
+
 The `main.js` should create windows and handle system events, a typical
 example being: