add completions generation example (#169)

examples/gen_completions.rs

@@ -0,0 +1,29 @@
+// Copyright 2019-present structopt developers
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[macro_use]
+extern crate structopt;
+
+use structopt::clap::Shell;
+use structopt::StructOpt;
+
+#[derive(StructOpt, Debug)]
+#[structopt(name = "example", about = "An example of StructOpt usage.")]
+struct Opt {
+    /// A flag, true if used in the command line.
+    #[structopt(short = "d", long = "debug", help = "Activate debug mode")]
+    debug: bool,
+}
+
+fn main() {
+    // generate `bash` completions in "target" directory
+    Opt::clap().gen_completions(env!("CARGO_PKG_NAME"), Shell::Bash, "target");
+
+    let opt = Opt::from_args();
+    println!("{:?}", opt);
+}