Add docs for prettify.

site/dev/tools/markdown.md

Index: site/dev/tools/markdown.md
diff --git a/site/dev/tools/markdown.md b/site/dev/tools/markdown.md
index e08ccb9473e798b1057144c44b27b728129ce228..66feb9460e65cf285e3063237a74ed81848855da 100644
--- a/site/dev/tools/markdown.md
+++ b/site/dev/tools/markdown.md
@@ -42,8 +42,14 @@ Or go old school and use [ASCII art](http://asciiflow.com/):
 
 ~~~~
 
-Format code snippets or other preformatted text.
+Format code snippets or other preformatted text. Just surround the code
+with `~~~~`. You can also trigger syntax highlighting by putting in
+the following HTML comment before the code section:
 
+    <!--?prettify?-->
+
+
+<!--?prettify?-->
 ~~~~
 class SK_API SkPaint {
 public:
@@ -54,6 +60,7 @@ public:
     SkPaint& operator=(const SkPaint&);
 ~~~~
 
+
 Tables
 
   Name    | Value    | Summary