Added from_crawler to middleware docs

docs/topics/downloader-middleware.rst

@@ -157,6 +157,17 @@ more of the following methods:
       :param spider: the spider for which this request is intended
       :type spider: :class:`~scrapy.spiders.Spider` object
 
+   .. method:: from_crawler(cls, crawler)
+    
+      If present, this classmethod is called to create a middleware instance
+      from a :class:`~scrapy.crawler.Crawler`. It must return a new instance
+      of the middleware. Crawler object provides access to all Scrapy core
+      components like settings and signals; it is a way for middleware to
+      access them and hook its functionality into Scrapy.
+   
+      :param crawler: crawler that uses this middleware
+      :type crawler: :class:`~scrapy.crawler.Crawler` object
+
 .. _topics-downloader-middleware-ref:
 
 Built-in downloader middleware reference

docs/topics/spider-middleware.rst

@@ -164,6 +164,17 @@ following methods:
         :param spider: the spider to whom the start requests belong
         :type spider: :class:`~scrapy.spiders.Spider` object
 
+    .. method:: from_crawler(cls, crawler)
+    
+       If present, this classmethod is called to create a middleware instance
+       from a :class:`~scrapy.crawler.Crawler`. It must return a new instance
+       of the middleware. Crawler object provides access to all Scrapy core
+       components like settings and signals; it is a way for middleware to
+       access them and hook its functionality into Scrapy.
+    
+       :param crawler: crawler that uses this middleware
+       :type crawler: :class:`~scrapy.crawler.Crawler` object
+
 
 .. _Exception: https://docs.python.org/2/library/exceptions.html#exceptions.Exception