Mention that it does not work on Windows platform

[blaise-io]

I built this project to improve my workflow with Django and Grunt. I work in a *nix environment, so I didn't bother with trying to support other platforms.

I tried anyway, and got the error below. Maybe mention in the readme that it does not work on Windows so other people won't have to try.

(env1) D:\Sites\storycollective\app>python manage.py grunt
Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "D:\Sites\storycollective\env1\lib\site-packages\django\core\management\__init__.py", line 399, in execute_from_command_line
    utility.execute()
  File "D:\Sites\storycollective\env1\lib\site-packages\django\core\management\__init__.py", line 392, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "D:\Sites\storycollective\env1\lib\site-packages\django\core\management\base.py", line 242, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "D:\Sites\storycollective\env1\lib\site-packages\django\core\management\base.py", line 285, in execute
    output = self.handle(*args, **options)
  File "D:\Sites\storycollective\env1\lib\site-packages\commando\management\base.py", line 302, in handle
    validate(*arguments, **options)
  File "D:\Sites\storycollective\env1\lib\site-packages\grunt\management\grunt.py", line 44, in validate_grunt
    self.check_program("grunt")
  File "D:\Sites\storycollective\env1\lib\site-packages\commando\management\base.py", line 362, in check_program
    if not check_program(name):
  File "D:\Sites\storycollective\env1\lib\site-packages\commando\management\base.py", line 131, in check_program
    subprocess.check_call(("which", name), stdout=null, stderr=null)
  File "C:\Python27\Lib\subprocess.py", line 535, in check_call
    retcode = call(*popenargs, **kwargs)
  File "C:\Python27\Lib\subprocess.py", line 522, in call
    return Popen(*popenargs, **kwargs).wait()
  File "C:\Python27\Lib\subprocess.py", line 709, in __init__
    errread, errwrite)
  File "C:\Python27\Lib\subprocess.py", line 957, in _execute_child
    startupinfo)
WindowsError: [Error 2] The system cannot find the file specified