udpate memusage docs

Author: dangra

docs/topics/extensions.rst

@@ -203,7 +203,8 @@ Memory usage extension
 
 .. class:: scrapy.extensions.memusage.MemoryUsage
 
-.. note:: This extension does not work in Windows.
+.. note:: psutil_ package is highly recommended, but also required to use this
+          extension in Windows.
 
 Monitors the memory used by the Scrapy process that runs the spider and:
 
@@ -223,6 +224,8 @@ can be configured with the following settings:
 * :setting:`MEMUSAGE_NOTIFY_MAIL`
 * :setting:`MEMUSAGE_REPORT`
 
+.. _psutil: https://pypi.python.org/pypi/psutil
+
 Memory debugger extension
 ~~~~~~~~~~~~~~~~~~~~~~~~~