Add spf13's pflag library

README.md

@@ -144,6 +144,7 @@ Please take a quick gander at the [contribution guidelines](https://github.com/a
 * [liner](https://github.com/peterh/liner) - A Go readline-like library for command-line interfaces.
 * [mitchellh/cli](https://github.com/mitchellh/cli) - A Go library for implementing command-line interfaces.
 * [mow.cli](https://github.com/jawher/mow.cli) - A Go library for building CLI applications with sophisticated flag and argument parsing and validation.
+* [pflag](https://github.com/spf13/pflag) - Drop-in replacement for Go's flag package, implementing POSIX/GNU-style --flags.
 * [readline](https://github.com/chzyer/readline) - A pure golang implementation that provide most of features in GNU-Readline under MIT license.
 * [sflags](https://github.com/octago/sflags) - Struct based flags generator for flag, urfave/cli, pflag, cobra, kingpin and other libraries.
 * [ukautz/clif](https://github.com/ukautz/clif) - A small command line interface framework.