Additional Windows build notes #76

It may be helpful to add the following notes to the Windows build guide:

• You need to explicitly run the 64-bit version of msys2 (at least on a 64-bit system)
• If you encounter errors when running python3 setup.py install for the pyfastogt python scripts you may need to run this command python -m pip install -U --force-reinstall pip setuptools
• If you encounter errors with python3 build_env.py you may need to exit all msys2 consoles, kill and msys2 processes that remain running and run the following from a standard windows command prompt (does not need to be elevated): c:\msys32\usr\bin\dash.exe c:\msys32\usr\bin\rebaseall -v