Make #session raise error when store is not configured

Add more explanation on sessions in readme

lib/telegram/bot/updates_controller/session.rb

@@ -8,19 +8,26 @@ module Telegram
       module Session
         extend ActiveSupport::Concern
 
+        module ClassMethods
+          # Builds session with given key and optional store (default to session_store).
+          # This way it's easier to define multiple custom sessions,
+          # ex. one for group chat and one for user.
+          def build_session(key, store = session_store)
+            raise 'session_store is not configured' unless store
+            key ? SessionHash.new(store, key) : NullSessionHash.new
+          end
+        end
+
         def process_action(*)
           super
         ensure
-          session.commit
+          session.commit if @_session
         end
 
         protected
 
         def session
-          @_session ||= begin
-            key = session_key
-            key ? SessionHash.new(self.class.session_store, key) : NullSessionHash.new
-          end
+          @_session ||= self.class.build_session(session_key)
         end
 
         def session_key