Golang Document enhance: Proxy setting if you run go get behind firewall

[zixu mo]

Shall we improve golang official document, guide people how to set the proxy behind firewall.

If your company or your network can not reach golang.org, you may fail to run go get to download and install the packages from golang.org

But we can set up proxy setting on linux/windows

For example

root@ed27c545f7af:~# cat ~/proxy.conf
export http_proxy=http://172.17.42.1:8118
export https_proxy=$http_proxy
export ftp_proxy=$http_proxy
export rsync_proxy=$http_proxy
export no_proxy="localhost,127.0.0.1,localaddress,.localdomain.com"