add explicit licensing file

Author: krichprollsch

LICENSING.md

@@ -0,0 +1,13 @@
+# Licensing
+
+License names used in this document are as per [SPDX License
+List](https://spdx.org/licenses/).
+
+The default license for this project is [Apache-2.0](LICENSE).
+
+The following directories and their subdirectories are licensed under their
+original upstream licenses:
+
+```
+vendor/
+```