Clarify Prettier defaulting behavior (#616)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

readme.md

@@ -221,14 +221,23 @@ Default: `false`
 
 Format code with [Prettier](https://github.com/prettier/prettier).
 
-The [Prettier options](https://prettier.io/docs/en/options.html) will be read from the [Prettier config](https://prettier.io/docs/en/configuration.html) and if **not set** will be determined as follow:
+[Prettier options](https://prettier.io/docs/en/options.html) will be based on your [Prettier config](https://prettier.io/docs/en/configuration.html). XO will then **merge** your options with its own defaults:
 - [semi](https://prettier.io/docs/en/options.html#semicolons): based on [semicolon](#semicolon) option
 - [useTabs](https://prettier.io/docs/en/options.html#tabs): based on [space](#space) option
 - [tabWidth](https://prettier.io/docs/en/options.html#tab-width): based on [space](#space) option
 - [trailingComma](https://prettier.io/docs/en/options.html#trailing-commas): `all`
 - [singleQuote](https://prettier.io/docs/en/options.html#quotes): `true`
 - [bracketSpacing](https://prettier.io/docs/en/options.html#bracket-spacing): `false`
-- [bracketSameLine](https://prettier.io/docs/en/options.html#bracket-line): `false`
+
+To stick with Prettier's defaults, add this to your Prettier config:
+
+```js
+module.exports = {
+	trailingComma: 'es5',
+	singleQuote: false,
+	bracketSpacing: true,
+};
+```
 
 If contradicting options are set for both Prettier and XO an error will be thrown.