title: Symfony/Doctrine entity mapping configuration
body: <p>I'm studying about creating custom bundles in Symfony 4.1. I have a simple bundle working with a simple controller and a route /hello. I want to create some Doctrine Entities to be used by the external app that includes my bundle. How can I do this configuration and entities mapping? Following my current folder structure:</p>
  
  <pre><code>- custom-hello-bundle
      - Controller
          HelloController.php
      - DependencyInjection
          Configuration.php
          CustomHelloExtension.php
      - Resources
          - config
              routes.xml
              services.xml
      - Tests
      composer.json
      CustomHelloBundle.php
      phpunit.xml.dist
  </code></pre>
  
  <p>I know that I have to create an Entity folder, and add an entity called Hello.php (for example). But how do I configure the Doctrine to mapping this entity inside this custom bundle?</p>