Document how header row is populated

when using a collection of data objects

See #246

docs.html

@@ -270,6 +270,7 @@ var csv = Papa.unparse([
 
 					<div class="grid-33">
 						<pre><code class="language-javascript">// With implicit header row
+// (keys of first object populate header row)
 var csv = Papa.unparse([
 	{
 		"Column 1": "foo",
@@ -804,4 +805,4 @@ var csv = Papa.unparse({
 		</footer>
 
 	</body>
-</html>
+</html>