Add LICENSE.txt

LICENSE.txt

+Copyright 2020 Stichting Nederlandse Wetenschappelijk Onderzoek Instituten,
+               ASTRON Netherlands Institute for Radio Astronomy
+
+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.