pywin32 is required by Twisted. closes scrapy#937

see:
* http://twistedmatrix.com/trac/ticket/6032
* https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2028

Author: dangra

docs/intro/install.rst

@@ -54,6 +54,10 @@ Windows
 
       python --version
 
+* Install `pywin32` from http://sourceforge.net/projects/pywin32/
+
+  Be sure you download the architecture (win32 or amd64) that matches your system
+
 * Install `pip`_ from https://pip.pypa.io/en/latest/installing.html
 
   Now open a Command prompt to check ``pip`` is installed correctly::