Create example.dart

The pub site can show example code for a package.
https://www.dartlang.org/tools/pub/package-layout#examples

related issue : dart-lang/site-www#413
diff --git a/example/example.dart b/example/example.dart
new file mode 100644
index 0000000..4089a3d
--- /dev/null
+++ b/example/example.dart
@@ -0,0 +1,6 @@
+import 'package:boolean_selector/boolean_selector.dart';
+
+void main(List<String> args) {
+  var selector = new BooleanSelector.parse("(x && y) || z");
+  print(selector.evaluate((variable) => args.contains(variable)));
+}