added a touch more information to the apache configuration instructions

In the text there is a section about how to serve the assets with
far-future headers. The snippet that is proposed doesn't work on Ubuntu
11 (probably others) as mod_expires is not enabled out-of-the-box. I
noted this, and gave an example of how to load mod_expires with a2enmod.

guides/source/asset_pipeline.textile

@@ -473,6 +473,9 @@ Precompiled assets exist on the filesystem and are served directly by your web s
 For Apache:
 
 <plain>
+# the following requires mod_expires
+# on Ubuntu issue:
+# sudo a2enmod expires
 <LocationMatch "^/assets/.*$">
   # Use of ETag is discouraged when Last-Modified is present
   Header unset ETag