Add an example with at least 2 arguments

Author: TeXitoi

examples/at_least_two.rs

@@ -0,0 +1,15 @@
+#[macro_use]
+extern crate structopt;
+
+use structopt::StructOpt;
+
+#[derive(StructOpt, Debug)]
+struct Opt {
+    #[structopt(raw(required = "true", min_values = "2"))]
+    foos: Vec<String>,
+}
+
+fn main() {
+    let opt = Opt::from_args();
+    println!("{:?}", opt);
+}