Updated documentation of spider arguments to include required super call.

docs/topics/spiders.rst

@@ -61,9 +61,10 @@ Spiders receive arguments in their constructors::
     class MySpider(BaseSpider):
         name = 'myspider'
 
-        def __init__(self, category=None):
+        def __init__(self, category=None, *args, **kwargs):
             self.start_urls = ['http://www.example.com/categories/%s' % category]
             # ...
+            super(MySpider, self).__init__(*args, **kwargs)
 
 Spider arguments can also be passed through the Scrapyd ``schedule.json`` API.
 See `Scrapyd documentation`_.