Update gamemode.ini (#490)

Clarify that user must be added to the gamemode group in order to make Core Parking work.

example/gamemode.ini

@@ -87,6 +87,8 @@ disable_splitlock=1
 ; a range. E.g "park_cores=1,8-15" would park cores 1 and 8 to 15.
 ; The default is uncommented is to disable parking but enable pinning. If either is enabled the code will
 ; currently only properly autodetect Ryzen 7900x3d, 7950x3d and Intel CPU:s with E- and P-cores.
+; For Core Parking, user must be added to the gamemode group (not required for Core Pinning):
+; sudo usermod -aG gamemode $(whoami)
 ;park_cores=no
 ;pin_cores=yes