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>