add license

LICENSE.md

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+The content is made available under the [Creative Commons Attribution 4.0](https://creativecommons.org/licenses) license. 
+
+The scripts and code are available under the [GNU General Public License v3.0](https://www.gnu.org/licenses).
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, version 3 of the License.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    We acknowledge the World Climate Research Program's Working Group on 
+    Coupled Modelling, which is responsible for CMIP, and we thank the 
+    climate modeling groups (listed in the Atlas inventory folders in this 
+    repository) for producing and making available their model output. 
+
+
+