title: 'error Command "encore" not found.' when running 'yarn run encore' in Symfony4
body: <p>I work in a Symfony 4.2 project, and try to use Bootstrap theming (so I need to compile Sass code). I followed the process given in the official doc, which indicates to use webpack-encore. But I have an error when using it accordingly to the doc.</p>
  
  <p>Here is the page indicating the process :
  <a href="https://symfony.com/doc/current/frontend/encore/simple-example.html" rel="nofollow noreferrer">https://symfony.com/doc/current/frontend/encore/simple-example.html</a></p>
  
  <p>So I installed Encore, and then I launch <code>yarn run encore</code>. And here is what it says in the terminal :</p>
  
  <pre><code>yarn run v1.16.0
  warning package.json: No license field
  error Command "encore" not found.
  info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
  </code></pre>
  
  <p>So I visited the site indicated in the error, which says :</p>
  
  <h2>You may define scripts in your package.json file.</h2>
  
  <p>And if I look in my package.json in the root directory, it indeed only says :</p>
  
  <pre><code>  "devDependencies": {
      "bootstrap": "^4.3.1",
      "jquery": "^3.4.1",
      "node-sass": "^4.12.0",
      "popper": "^1.0.1",
      "sass-loader": "^7.0.1"
    }
  }
  </code></pre>
  
  <p>There is no script for any encore command :-(</p>
  
  <p>However the rest seems good. Here is an extract of composer.json :</p>
  
  <pre><code>...
      "require": {
  ...
          "symfony/webpack-encore-bundle": "^1.5",
  ...
      },
  ...
  </code></pre>
  
  <p>And I have a webpack.config.js .</p>
  
  <p>I runned <code>composer require encore</code> and <code>yarn install</code> without any error.</p>
  
  <p>But nowhere in the Symfony doc I found anything about tweaking the package.json oO So how do I make it work ? Why doesn't the <code>yarn run encore</code> work ?</p>