Document possible build failure with broken PATH

We should document the fact that if the user has sh.exe or make.exe from Cygwin or a different in path, Qt Creator (and the Nokia Qt SDK) may not work.

Symtoms are errors including paths that seem to lack slashes (C:NokiaQtSDK...). Some embedded toolkits based on Cygwin seem to suggest it's a good thing to register their tool chain directories in the Path during installation, but in fact it's broken (and we do not assume that). The proper way for the user to remove the affecting directories from the PATH variable and create a new shell link which adds the required pathes (That's what Visual Studio and Qt do).

The problems may occur with the following environments:

• Qt SDK (uses MinGW for Desktop Qt)
• Nokia Qt SDK (uses MinGW for Simulator)
• any other Qt for MinGW version used with Creator.