Add signature for Jefté. relates to #822

Adds signature but does not address the content suggestions. If a more direct suggestion "This should be changed to say this" were made I could make that change too. Will leave that issue open.

src/index.html

@@ -1217,6 +1217,7 @@ <h3 id="signed-by">Signed by:</h3>
       <li>Mike Mai, designer &amp; coder</li>
       <li>Scott Werley, Frontend Designer/Developer and Accessibility Specialist, self</li>
       <li>Claire Annan, Consulting Software Engineer, 18F</li>
+      <li>Jefté Macêdo, UX researcher and Accessibility Specialist</li>
     </ol>
     <p>
       <a class="add-your-name" href="https://github.com/karlgroves/overlayfactsheet/#endorse-this-statement">