add build script file.

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

script/build.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+for GOOS in darwin linux windows; do
+  for GOARCH in 386 amd64; do
+    echo "Build: ${GOOS}, Arch: ${GOARCH}"
+    GOOS=$GOOS GOARCH=$GOARCH go build -ldflags="-w" -o bin/$GOOS/$GOARCH/gopush gopush.go;
+  done
+done