Add 'link opens in new tab' explanation

app/templates/views/documentation.html

@@ -25,6 +25,6 @@
     <p>A developer should be able to set up the API client and start sending test messages in around 30 minutes. A full integration can take a few days, depending on the other systems you’re using.</p>
   <h2 class="heading-medium">Integrate directly with the API</h2>
     <p>If you cannot use the client libraries, you can integrate directly with the API instead.</p>
-    <p>Read the REST API documentation.</p>
+    <p>Read the REST API documentation (this link opens in a new tab).</p>
 
 {% endblock %}