add cross link to data post

Author: railsagainstignorance

_posts/2018-03-23-crosswords.html

@@ -83,3 +83,6 @@ <h3>Generating new grids</h3>
 
 <h2>Available now!</h2>
 <p>Please try the latest interactive crosswords at <a href="https://labs.ft.com/crosswords/" target="_blank">labs.ft.com/crosswords/</a> and let us know what you think.</p>
+
+<h2>Addendum</h2>
+<p>For more detail about the underlying crossword data format used in this experiment, see the <a href="/experiment/2018/03/26/crossword-data-format.html" target="_blank">following post</a></p>