doc: How to get cgo to work on windows 64 bit?

[alexbrainman]

See https://www.reddit.com/r/golang/comments/60xr8t/how_to_get_cgo_to_work_on_windows_64_bit/ for details.

Maybe we can be more helpful here. Maybe better documentation. Maybe we could provide more precise instructions about installing gcc on windows.

Thank you.

Alex

[brandonryan]

Took a shot at this as first contribution. Please let me know if it needs anymore changes.
In the future it might be a good idea to have a "supported cgo compilers" page, because I know that msvc support is being worked on. I've also had success with http://tdm-gcc.tdragon.net/

[gopherbot]

Change https://golang.org/cl/173997 mentions this issue: doc: clarify to use MinGW-W64 for amd64 systems