title: Wordpress: save custom billing field in account Billing address
body: <p>Using Wordpress/Woocommerce.</p>
  
  <p>I have this code which adds a custom billing field in checkout page called: "NIF/CIF". It works fine but its value it not saved in the customer account "Billing Address" data.</p>
  
  <p>Once the customer makes a first order all billing address values are saved in his account: Address, State, Country, etc. But my custom field is not saved.  </p>
  
  <p>I guess that in my function is missing a line of code to save its value in the database, but I don't know how to start with this.</p>
  
  <pre><code>/*******************************
      CUSTOM BILLING FIELD
  *********************************/
  add_filter('woocommerce_billing_fields', 'custom_woocommerce_billing_fields');
  
  function custom_woocommerce_billing_fields($fields)
  {
  
      $fields['nif_cif'] = array(
          'label' =&gt; __('NIF/CIF', 'woocommerce'), // Add custom field label
          'placeholder' =&gt; _x('NIF/CIF', 'placeholder', 'woocommerce'), // Add custom field placeholder
          'required' =&gt; true, // if field is required or not
          'clear' =&gt; false, // add clear or not
          'type' =&gt; 'text', // add field type
          'class' =&gt; array('my-css')    // add class name
      );
  
      return $fields;
  }
  </code></pre>