Add deadlines on 'how to vote' guidance

This is to bring metadata in line with upcoming content changes

config/machine_readable/how-to-vote.yml

@@ -65,6 +65,10 @@ faqs:
         <li>permanently</li>
       </ul>
       <p>Arrange to <a href="/how-to-vote/voting-by-proxy?src=schema">vote by proxy</a> if there are under 2 weeks until election day and you have not made arrangements.</p>
+      <p>There’s a different form to <a href="https://www.eoni.org.uk/Vote/Voting-by-post-or-proxy">apply to vote by post in Northern Ireland</a>.</p>
+      <h3 id="postal-vote-deadlines-for-6-may-elections">Postal vote deadlines for 6 May elections</h3>
+      <p>If you’re registered to vote in England or Wales, your application form must arrive at your local Electoral Registration Office by 20 April at 5pm if you want to vote by post on 6 May 2021.</p>
+      <p>If you’re registered to vote in Scotland, you can no longer apply for a postal vote for the 6 May 2021 elections. You can still apply for a proxy vote.</p>
 
   - question: Voting by proxy
     answer: >
@@ -78,6 +82,7 @@ faqs:
       <h2 id="how-to-apply-for-a-proxy-vote">How to apply for a proxy vote</h2>
       <p><a href="/government/collections/proxy-voting-application-forms?src=schema">Apply for a proxy vote</a> using a paper form. You need to send it to your local <a href="/contact-electoral-registration-office?src=schema">Electoral Registration Office</a>.</p>
       <p>Usually, you need to apply for a proxy vote at least 6 working days before election day if you want to vote in England, Scotland or Wales.</p>
+      <p>If you want to vote by proxy on 6 May 2021, your application form must arrive at your local Electoral Registration Office by 27 April at 5pm.</p>
 
   - question: Voting from abroad
     answer: >