title: how to bind a string with mat-checkbox
body: <p>I am using some mat-checkbox in my app and i do not know how to receive a string from the checkbox instead a boolean.</p>
  
  <p>Reading the API in the website
  <a href="https://material.angular.io/components/checkbox/api" rel="nofollow noreferrer">Angular Material checkbox API </a>
   I could find the 'value' propertie that can assing a string to this checkbox... but how can i bind this to my variable? </p>
  
  <p>I am trying something like this.</p>
  
  <pre><code>&lt;mat-checkbox color="warn" value="Hello" [(ngModel)]="myString"&gt; Hello &lt;/mat-checkbox&gt;
  </code></pre>
  
  <p>but my variable is getting a boolean... how can i do it?</p>