add readme

README.md

@@ -4,4 +4,22 @@ A flutter package for showing a country code selector.
 
 ## Usage
 
- 
+Just put the component in your application setting the onChanged callback.
+
+ ```dart
+ ......
+ @override
+  Widget build(BuildContext context) => new Scaffold(
+      body: new Center(
+        child: new CountryCodePicker(
+          onChanged: print,
+        ),
+      ));
+.....
+ ```
+
+ It's also possible to select the initial selection using the country code.
+
+## Known issues
+
+- On iOS the icons are not showned