Doument POSIX path translation in MSYS2

[mbland]

Adding the following to go-core.bash would avoid some nasty surprises:

if [[ "$OSTYPE" == 'msys' ]]; then
  export MSYS_NO_PATHCONV='true'
  export MSYS2_ARG_CONV_EXCL='*'
fi

per: How to stop mingw and msys from mangling path names given at the command line? (Stack Overflow)

This may have ramifications for the ./go get command, though I'm not sure yet.

[mbland]

Per mbland/url-pointers#62, setting this at the top level actually breaks some programs, notably npm install. So rather than automatically disabling it, it should be better documented somewhere.