Document supported GOOS values in README (FreeBSD is not supported). #770
Description
I tried to integrate a gopherjs build step into our jenkins CI, which is running on FreeBSD.
The same command runs fine in my dev environment under Linux.
[staging ~]$ gopherjs build -o viewer.js proj/viewer
../../local/go/src/vendor/golang_org/x/net/route/address.go:25:42: undeclared name: sysAF_LINK
../../local/go/src/vendor/golang_org/x/net/route/address.go:42:9: undeclared name: sysAF_LINK
../../local/go/src/vendor/golang_org/x/net/route/address.go:64:35: undeclared name: sysAF_LINK
../../local/go/src/vendor/golang_org/x/net/route/address.go:124:43: undeclared name: sysAF_INET
../../local/go/src/vendor/golang_org/x/net/route/address.go:127:9: undeclared name: sizeofSockaddrInet
../../local/go/src/vendor/golang_org/x/net/route/address.go:127:37: undeclared name: sizeofSockaddrInet
../../local/go/src/vendor/golang_org/x/net/route/address.go:136:9: undeclared name: sysAF_INET
../../local/go/src/vendor/golang_org/x/net/route/address.go:148:43: undeclared name: sysAF_INET6
../../local/go/src/vendor/golang_org/x/net/route/address.go:151:9: undeclared name: sizeofSockaddrInet6
../../local/go/src/vendor/golang_org/x/net/route/address.go:151:38: undeclared name: sizeofSockaddrInet6
../../local/go/src/vendor/golang_org/x/net/route/address.go:151:38: too many errors
Would like to supply more info but I'm not even sure where these paths come from..
I'm using Go 1.10 and GopherJS 1.10-2.