Update example to use Bundler (#97)

Since Rubocop has multiple gems, it's best to rely on Bundler to install them rather than not. Other popular gems like Rails use this same feature to optimize their rubocop workflows. https://github.com/rails/rails/blob/main/.github/workflows/rubocop.yml

README.md

@@ -100,30 +100,44 @@ Optional. Run Rubocop with bundle exec. Default: `false`.
 
 ## Example usage
 
-You can create [RuboCop Configuration](https://docs.rubocop.org/rubocop/configuration.html) and this action uses that config too.
+This action will use your [RuboCop Configuration](https://docs.rubocop.org/rubocop/configuration.html) automatically.
+
+In your `Gemfile`, ensure all Rubocop gems are in a named (e.g. rubocop) group:
+
+```ruby
+group :development, :rubocop do
+  gem 'rubocop', require: false
+  gem 'rubocop-rails', require: false
+  # ...
+end
+```
+
+Create the following workflow. The `BUNDLE_ONLY` environment variable will tell Bundler to only install the specified group.
 
 ```yml
 name: reviewdog
-on: [pull_request]
+on:
+  pull_request:
 permissions:
   contents: read
   pull-requests: write
 jobs:
   rubocop:
     name: runner / rubocop
     runs-on: ubuntu-latest
+    env:
+      BUNDLE_ONLY: rubocop
     steps:
-      - name: Check out code
-        uses: actions/checkout@v4
+      - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: 3.0.0
-      - name: rubocop
-        uses: reviewdog/action-rubocop@v2
+          ruby-version: '3.3'
+          bundler-cache: true
+      - uses: reviewdog/action-rubocop@v2
         with:
-          rubocop_version: gemfile
-          rubocop_extensions: rubocop-rails:gemfile rubocop-rspec:gemfile
           reporter: github-pr-review # Default is github-pr-check
+          skip_install: true
+          use_bundler: true
 ```
 
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