add build instruction for windows and mac

.gitignore

 .*.sw[op]
 .DS_Store
-acme-downloader
\ No newline at end of file
+acme-downloader
+acme-downloader.exe
\ No newline at end of file

README.md

@@ -51,4 +51,22 @@ Debian/RedHat/Arch OS families
 
 ## build
 
+### From Linux for Linux
+
 you can use `build.sh` from this repo
+
+### From Linux for Windows
+
+```bash
+PROG_VERSION="1.0"
+BUILDTIME=$(date -u '+%Y-%m-%d_%H:%M:%S')
+env GOOS=windows GOARCH=amd64 go build -ldflags "-s -w -X main.appVersion=${PROG_VERSION} -X main.buildTime=${BUILDTIME}" .
+```
+
+### From Linux for MacOS (not tested)
+
+```bash
+PROG_VERSION="1.0"
+BUILDTIME=$(date -u '+%Y-%m-%d_%H:%M:%S')
+env GOOS=darwin GOARCH=amd64 go build -ldflags "-s -w -X main.appVersion=${PROG_VERSION} -X main.buildTime=${BUILDTIME}" .
+```

build.sh

 #!/bin/bash
+#
+# for windows and Mac check the README
+#
 BIN_NAME=acme-downloader
 PATH=$PATH:$(go env GOPATH)/bin
 GOPATH=$(go env GOPATH)