Add NOTICE file

NOTICE

@@ -0,0 +1,14 @@
+Copyright 2018-present HiveMQ GmbH
+
+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use the contents of this repository 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.