Docs: Clarify package.json requirement in Getting Started (refs #13549)

docs/user-guide/getting-started.md

@@ -20,7 +20,7 @@ npm install eslint --save-dev
 yarn add eslint --dev
 ```
 
-You should then set up a configuration file:
+You should then set up a configuration file, and the easiest way to do that is to use the `--init` flag:
 
 ```
 $ npx eslint --init
@@ -30,6 +30,8 @@ $ npx eslint --init
 $ yarn run eslint --init
 ```
 
+**Note:** `--init` assumes you have a `package.json` file already. If you don't, make sure to run `npm init` or `yarn init` beforehand.
+
 After that, you can run ESLint on any file or directory like this:
 
 ```