Update readme for Wnck

[khaitranhq]

Following guide in readme, I got this error

Traceback (most recent call last):                                                                                                                                                                                 
  File "/home/lewis/Downloads/XFCE-Dynamic-Workspace/./dynamic_workspaces.py", line 4, in <module>                                                                                                                 
    gi.require_version('Wnck', '3.0')                                                                                                                                                                              
  File "/usr/lib/python3/dist-packages/gi/__init__.py", line 126, in require_version                                                                                                                               
    raise ValueError('Namespace %s not available' % namespace)                                                                                                                                                     
ValueError: Namespace Wnck not available

After searching google, I see that we should install gir1.2-wnck-3.0 on Ubuntu/Debian to run the script.
Please check again and update Readme if needed.
Reference: ssokolow/quicktile#95 (comment)