add flag library.

README.md

@@ -141,6 +141,7 @@ Please take a quick gander at the [contribution guidelines](https://github.com/a
 * [cobra](https://github.com/spf13/cobra) - A Commander for modern Go CLI interactions
 * [docopt.go](https://github.com/docopt/docopt.go) - A command-line arguments parser that will make you smile.
 * [drive](https://github.com/odeke-em/drive) - Google Drive client for the commandline
+* [flag](https://github.com/cosiner/flag) - A simple but powerful command line option parsing library for Go support subcommand
 * [go-arg](https://github.com/alexflint/go-arg) - Struct-based argument parsing in Go
 * [go-flags](https://github.com/jessevdk/go-flags) - go command line option parser
 * [kingpin](https://github.com/alecthomas/kingpin) - A command line and flag parser supporting sub commands.