Add section to pipeline docs to help people upgrading

These are the options from a brand new app, and
should help people get it right when upgrading.

railties/guides/source/asset_pipeline.textile

@@ -434,3 +434,50 @@ A good example of this is the +jquery-rails+ gem which comes with Rails as the s
 h3. Making Your Library or Gem a Pre-Processor
 
 TODO: Registering gems on "Tilt":https://github.com/rtomayko/tilt enabling Sprockets to find them.
+
+h3. Upgrading from Old Versions of Rails
+
+There are two issues when upgrading. The first is moving the files to the new locations. See the section above for guidance on the correct locations for different file types. 
+
+The second is updating the various environment files with the correct default options. The following changes reflect the defaults in version 3.1.0.
+
+In +application.rb+:
+
+<erb>
+# Enable the asset pipeline
+config.assets.enabled = true
+
+# Version of your assets, change this if you want to expire all your assets
+config.assets.version = '1.0'
+</erb>
+
+In +development.rb+:
+
+<erb>
+# Do not compress assets
+config.assets.compress = false
+
+# Expands the lines which load the assets
+config.assets.debug = true
+</erb>
+
+And in +production.rb+:
+
+<erb>
+# Compress JavaScripts and CSS
+config.assets.compress = true
+
+# Don't fallback to assets pipeline if a precompiled asset is missed
+config.assets.compile = false
+
+# Generate digests for assets URLs
+config.assets.digest = true
+
+# Defaults to Rails.root.join("public/assets")
+# config.assets.manifest = YOUR_PATH
+
+# Precompile additional assets (application.js, application.css, and all non-JS/CSS are already added)
+# config.assets.precompile += %w( search.js )
+</erb>
+
+There are no changes to +test.rb+.