add import line to dart usage code block (#94)

Author: mp-v2

flutter_map_compass/README.md

@@ -26,7 +26,12 @@ dependencies:
 ```
 
 ## Usage
+To import the package
+```dart
+import 'package:flutter_map_compass/flutter_map_compass.dart';
+```
 
+To use the MapCompass
 ```dart
 @override
 Widget build(BuildContext context) {
@@ -57,4 +62,4 @@ Widget build(BuildContext context) {
 If you need help you
 can [open an issue](https://github.com/josxha/flutter_map_plugins/issues/new/choose)
 or join
-the [`flutter_map` discord server](https://discord.gg/BwpEsjqMAH).
+the [`flutter_map` discord server](https://discord.gg/BwpEsjqMAH).