title: Ionic button showing 'ion-button' is not a known element
body: <p>I am new to ionic, it seems like a silly question but I need some help
  Using some simple button is throwing error. I am using ionic 4.0. </p>
  
  <blockquote>
    <p>'ion-button' is not a known element:
    1. If 'ion-button' is an Angular component, then verify that it is part of this module.
    2. If 'ion-button' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message.</p>
  </blockquote>
  
  <pre><code>&lt;ion-button color="primary"&gt;Primary&lt;/ion-button&gt;
  </code></pre>