add spoiler extension to spec

Author: QuietMisdreavus

test/spec.txt

@@ -7734,6 +7734,26 @@ new paragraph~~.
 
 </div>
 
+<div class="extension">
+
+## Spoiler text (extension)
+
+Swift-cmark GFM provides the `spoiler` extension, providing an additional emphasis
+type for text that should be hidden and revealed by user interaction.
+
+Spoiler text is wrapped in two pipes (`|`).
+
+As the use of spoiler text depends on user interaction, GFM only emits a generic
+`<span>` tag that must be styled by the client.
+
+```````````````````````````````` example spoiler
+Spoiler alert: ||Hello, world!||
+.
+<p>Spoiler alert: <span class="spoiler">Hello, world!</span></p>
+````````````````````````````````
+
+</div>
+
 ## Links
 
 A link contains [link text] (the visible text), a [link destination]