added license header for src. Added changelog

lib/dart_di.dart

@@ -1,3 +1,16 @@
+/**
+ * Copyright 2021 Sergey Penkovsky <sergey.penkovsky@gmail.com>
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
 library dart_di;
 
 export 'package:dart_di/scope.dart';