windows: Add README with basic cross-compile instructions.
|This is experimental, community-supported Windows port of MicroPython.|
|It is based on Unix port, and expected to remain so.|
|To cross-compile under Debian/Ubuntu Linux system:|
|sudo apt-get install mingw32 mingw32-binutils mingw32-runtime|
|The port requires additional testing, debugging, and patches. Please|
|consider to contribute.|