Added some comments to the nginx config relating to expires.

nginx.conf

@@ -104,7 +104,14 @@ http {
     # Custom 404 page
     error_page 404 /404.html;
 
-    expires 1M;
+    # This is pretty long expiry and assume your using
+    # cachebusting with query params like
+    #   <script src="application.js?20110529">
+    #
+    # Just be careful if your using this on a frequently
+    # updated static site. You may want to crank this back
+    # to 5m which is 5 minutes.
+    expires 1M; # yes one month
 
     # Static assets
     location ~* ^.+\.(manifest|appcache)$ {