Update Linux usage instructions

README.md

 no IVI (Interchangeable Virtual Instrument) and no Rigol drivers are required.
 Python 2 is required. Python 3 is not supported.
 
-Tested only with Windows 7, Python 2.7.9 and Rigol DS1104Z (a fully upgraded DS1054Z oscilloscope).
+Tested with Windows 7, Python 2.7.9 and Rigol DS1104Z (a fully upgraded DS1054Z oscilloscope).
 
-Tested in a VMware machine with Linux Debian 7.7.0, Python 2.7.3 and PIL. 
-Python and PIL were already present in the default Debian installation.
+Tested in a VMware machine with Linux Debian 7.7.0, Python 2.7.3 and PIL.
 
 Windows Usage:
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 Linux Usage:
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+Python and PIL are already present in the default Debian 7.7.0 installation, 
+so it is not necessary to manually install them.
 Download the project from GitHub, open a Terminal, 
 go to the project's directory and run "OscScreenGrabLAN.py".