message: Add link to guide on postcode validation in documentation
diff: (meaning valid or invalid respectively)
  </p>
  <pre><h4><div class="http-method">GET</div>https://api.postcodes.io/postcodes/:postcode/validate</h4></pre>
  + <p>View our guide on <a href="https://ideal-postcodes.co.uk/guides/postcode-validation">postcode validation</a> to see a more detailed explanation</p>
  <h3 id="Nearest-Postcode">Nearest Postcode</h3>
  <p>Returns nearest postcodes for a given postcode.</p>
  <pre><h4><div class="http-method">GET</div>https://api.postcodes.io/postcodes/:postcode/nearest</h4></pre>
  