Update README

- add complex type flag reading example

Author: opcoder0

README.md

@@ -55,3 +55,80 @@ fn main() {
     println!("number of retries: {}", num_retries);
 }
 ```
+
+More complex types can be passed in as flags too as long as they implement `FromStr` and `Clone` traits. Example -
+
+```
+extern crate flags;
+
+use flags::{Flag, FlagSet};
+use std::env;
+use std::num::ParseIntError;
+use std::process;
+use std::str::FromStr;
+
+#[derive(Debug, PartialEq)]
+struct Point {
+    x: i32,
+    y: i32,
+}
+
+impl Clone for Point {
+    fn clone(&self) -> Self {
+        let x = self.x;
+        let y = self.y;
+        Self { x, y }
+    }
+}
+
+impl ToString for Point {
+    fn to_string(&self) -> String {
+        format!("({},{})", self.x, self.y)
+    }
+}
+
+impl FromStr for Point {
+    type Err = ParseIntError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let (x, y) = s
+            .strip_prefix('(')
+            .and_then(|s| s.strip_suffix(')'))
+            .and_then(|s| s.split_once(','))
+            .unwrap();
+
+        let x_fromstr = x.parse::<i32>()?;
+        let y_fromstr = y.parse::<i32>()?;
+
+        Ok(Point {
+            x: x_fromstr,
+            y: y_fromstr,
+        })
+    }
+}
+
+fn main() {
+    let point_flag = Flag::new(
+        Some("-p"),
+        Some("--point"),
+        "point on the 2d plane",
+        true,
+        Flag::kind::<Point>(),
+        Some(Box::new(Point { x: 0, y: 0 })),
+    );
+
+    let mut flagset = FlagSet::new();
+    flagset.add(&point_flag);
+    let result = flagset.parse(&mut env::args());
+    match result {
+        Err(e) => {
+            println!("{}", e);
+            println!("{}", flagset);
+            process::exit(1);
+        }
+        Ok(()) => {}
+    }
+    let coord = point_flag.borrow().get_value::<Point>().ok().unwrap();
+    println!("co-ordinates: {:?}", coord);
+}
+```