Add CC BY 4.0 license

LICENSE

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+Creative Commons Attribution 4.0 International License
+
+This work is licensed under the Creative Commons Attribution 4.0 International License.
+To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/ 
+or send a letter to Creative Commons, PO Box 1866, Mountain View, CA 94042, USA.
+
+You are free to:
+- Share — copy and redistribute the material in any medium or format
+- Adapt — remix, transform, and build upon the material for any purpose, even commercially
+
+Under the following terms:
+- Attribution — You must give appropriate credit, provide a link to the license, 
+  and indicate if changes were made. You may do so in any reasonable manner, 
+  but not in any way that suggests the licensor endorses you or your use.
+
+No additional restrictions — You may not apply legal terms or technological measures 
+that legally restrict others from doing anything the license permits.