Docs: Provide install commands for Yarn (#13661)

npx doesn't work properly with plugins installed with Yarn.

docs/user-guide/getting-started.md

@@ -24,12 +24,20 @@ You should then set up a configuration file:
 
 ```
 $ npx eslint --init
+
+# or
+
+$ yarn run eslint --init
 ```
 
 After that, you can run ESLint on any file or directory like this:
 
 ```
 $ npx eslint yourfile.js
+
+# or
+
+$ yarn run eslint yourfile.js
 ```
 
 It is also possible to install ESLint globally rather than locally (using `npm install eslint --global`). However, this is not recommended, and any plugins or shareable configs that you use must be installed locally in either case.