Update HISTORY log with version changes

HISTORY.md

@@ -1,6 +1,11 @@
 History of Changes
 ==================
 
+## v0.5.2 / 2012-06-21
+
+- Darken the content text color for better readability
+- Enhance navigation contrast colors
+
 ## v0.5.1 / 2012-04-29
 
 - Initial open-source release