Add instructions on updating the header package for PK

docs/prototype-kit-installation.md

@@ -78,4 +78,10 @@ The component can then be used like so:
     activeLinkId: 'servicelink3'
   }) }}
 {% endblock %}
-</code></pre>
+</code></pre>
+
+## How to update the header if you are using the Prototype Kit 
+
+You may not receive updates or bug fixes automatically. 
+
+To update the header to the most recent version, please run `npm update govuk-one-login-service-header` in your terminal.