Add apache 2 license

LICENSE

+Copyright 2020 Max Planck Institute for Software Systems
+
+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.
+
+------------------------------------------------------------------------------------
+
+This product bundles various third-party components under other open source licenses.  See licenses in the respective subdirectories.
+
+TODO: summarize third-party licenses here
\ No newline at end of file