add usage instructions

build.sh

 #!/bin/bash
 #
-# for windows and Mac check the README
+# you can choose your OS and Architecture as follows:
+# ./build.sh --os=windows --arch=amd64
+#
+# or you can use --help to check a list of platforms and options
 #
 if ! which go &>/dev/null; then
     echo "Go is not installed or is not in \$PATH"
     echo "giving up..."
-    exit
+    exit 1
 fi
 unset GOBIN
 BIN_NAME=acme-downloader
@@ -86,7 +89,7 @@ if [[ "$OS" == "windows" ]]; then
     BIN_NAME="${BIN_NAME}.exe"
 fi
 
-# env GOOS=windows GOARCH=amd64 go build -ldflags "-s -w -X main.appVersion=${PROG_VERSION} -X main.buildTime=${BUILDTIME}" .
+# env GOOS=$OS GOARCH=$ARCH go build -ldflags "-s -w -X main.appVersion=${PROG_VERSION} -X main.buildTime=${BUILDTIME}" .
 env GOOS=$OS GOARCH=$ARCH go get -ldflags "-s -w -X main.appVersion=${PROG_VERSION} -X main.buildTime=${BUILDTIME}" .
 if [ $? -gt 0 ]; then
     echo -e "\nthere was an error while compiling the code\n"