Add MapML 'Authoring' section

Author: Malvoz

spec/index.html

@@ -3153,6 +3153,16 @@ <h5>The <code>&lt;<dfn id="the-coordinates-element">coordinates</dfn>&gt;</code>
  </p>
  </section>
  </section>
+ 
+ <section class="informative">
+ <h3>Authoring</h3>
+ <p>
+ While MapML is human-readable and can be written by hand,
+ authors typically will use conversion programs
+ and other specialized software tools to generate MapML.
+ </p>
+ </section>
+ 
  <section>
  <h2 id="styling">Styling</h2>
  <p>MapML documents can be styled with Cascading Style Sheets.</p>