Add comments about why --notransparency is still needed

Thanks to Matthias Kretz kretz@kde.org for patch/info.

src/MainWindow.cpp

@@ -58,6 +58,11 @@
 
 using namespace Konsole;
 
+/* Normally it would be enough to just have this determined via the window
+   manager. But there exist GPU drivers (NVIDIA) that have serious performance
+   issues if transparency is enbled inside Konsole. The rest of the system
+   works fine. NVIDIA users might want to use --notransparency to work 
+   around such issues. */
 static bool useTransparency()
 {
     const KCmdLineArgs* args = KCmdLineArgs::parsedArgs();