README should clearly state MinGW support status

[asashnov]

Under Windows, Qt applications may be built with MSVC or MinGW compiler.
It's important to state which configurations are supported.

[manisandro]

gdb rather strongly implies that it works with gnu-toolchain produced binaries though?

[asashnov]

Well, I don't exactly know which debug symbol formats exists and what can understand which. So for me that wasn't obvious and worth mention.