more explicit comments for openid configuration

oa-openid/README.rdoc

@@ -21,7 +21,9 @@ Use the strategy as a middleware in your application:
 
     use OmniAuth::Strategies::OpenID, OpenID::Store::Filesystem.new('/tmp')
 
-Then simply direct users to '/auth/open_id' to prompt them for their OpenID identifier. You may also pre-set the identifier by passing an <tt>identifier</tt> parameter to the URL (Example: <tt>/auth/open_id?openid_url=google.com</tt>).
+Then simply direct users to '/auth/open_id' to prompt them for their OpenID identifier. You may also pre-set the identifier by passing an <tt>identifier</tt> parameter to the URL (Example: <tt>/auth/open_id?openid_url=yahoo.com</tt>).
+
+A list of all OpenID stores is available at http://github.com/openid/ruby-openid/tree/master/lib/openid/store/
 
 == OmniAuth Builder
 
@@ -35,3 +37,13 @@ If OpenID is one of several authentication strategies, use the OmniAuth Builder:
       provider :open_id, OpenID::Store::Filesystem.new('/tmp')
       provider :campfire
     end
+
+== Configured Identifiers
+
+You may pre-configure an OpenID identifier.  For example, to use Google's main OpenID endpoint:
+
+    use OmniAuth::Builder do
+      provider :openid, nil, :name => 'google', :identifier => 'https://www.google.com/accounts/o8/id'
+    end
+
+Note the use of nil, which will trigger ruby-openid's default Memory Store.