Config guide: add session store config option

railties/guides/source/configuring.textile

@@ -101,6 +101,14 @@ h4. Rails General Configuration
 
 * +config.serve_static_assets+ configures Rails to serve static assets. Defaults to _true_, but in the production environment is turned off. The server software used to run the application should be used to serve the assets instead.
 
+* +config.session_store+ is usually set up in +config/initializers/session_store.rb+ and specifies what class to use to store the session. Custom session stores can be specified like so:
+
+<ruby>
+  config.session_store = :my_custom_store
+</ruby>
+
+This custom store must be defined as +ActionDispatch::Session::MyCustomStore+.
+
 * +config.threadsafe!+ enables +allow_concurrency+, +cache_classes+, +dependency_loading+ and +preload_frameworks+ to make the application threadsafe.
 
 WARNING: Threadsafe operation is incompatible with the normal workings of development mode Rails. In particular, automatic dependency loading and class reloading are automatically disabled when you call +config.threadsafe!+.