Add copyright/license to readme

README.md

@@ -46,3 +46,17 @@ def init(_) do
   Supervisor.init(children, [strategy: :one_for_one])
 end
 ```
+
+## License
+
+Copyright (C) 2018-21 The Nerves Project Authors <[email protected]>
+
+    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](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.