title: I can not use the 'services.AddControllers().AddNewtonsoftJson()'
body: <p>I'm in Net core 3.1 and I'm trying to use <code>AddNewtonsoftJson()</code></p>
  <p>I'm getting:</p>
  <blockquote>
  <p>&quot;IMvcBuilder' does not contain a definition for 'AddNewtonsoftJson'
  and no accessible extension method 'AddNewtonsoftJson' accepting a
  first argument of type 'IMvcBuilder' could be found (are you missing a
  using directive or an assembly reference?)&quot;</p>
  </blockquote>
  <p>This is what I write in my ConfigureService:</p>
  <pre><code>services.AddControllers().AddNewtonsoftJson();
  </code></pre>
  <p>I already have installed NewtonSoft in my startup project.</p>