docs(readme): add usage section

Author: Zaba505

README.md

@@ -2,3 +2,36 @@
 
 `pflag` is a port of the [spf13](https://github.com/spf13/pflag)'s popular fork of the
 Go package by the same name.
+
+## Usage
+
+```rust
+use pflag::{FlagSet, Slice};
+use std::net::{IpAddr, Ipv4Addr};
+
+let mut flags = FlagSet::new("name");
+
+// Use higher level methods over add_flag directly.
+flags.int8("num", 0, "a flag for a number");
+flags.string_p("str", 's', String::from("default value"), "a flag for a String and has a shorthand");
+flags.ip_addr_slice(
+    "addrs",
+    Slice::from([IpAddr::V4(Ipv4Addr::new(0,0,0,0)), IpAddr::V4(Ipv4Addr::new(127,0,0,1))]),
+    "a multi-valued flag",
+);
+
+let args = "--num=1 -s world --addrs 192.168.1.1,192.168.0.1 --addrs=127.0.0.1 subcommand";
+if let Err(err) = flags.parse(args.split(' ')) {
+    panic!(err);
+}
+
+// Retrieving value is very easy with the value_of method.
+assert_eq!(*flags.value_of::<i8>("num").unwrap(), 1);
+assert_eq!(*flags.value_of::<String>("str").unwrap(), "world");
+assert_eq!(flags.value_of::<Slice<IpAddr>>("addrs").unwrap().len(), 3);
+
+// Any non-flag args i.e. positional args can be retrieved by...
+let args = flags.args();
+assert_eq!(args.len(), 1);
+assert_eq!(args[0], "subcommand");
+```