Add CC BY 4.0 license

README.md

@@ -633,3 +633,9 @@ Various resources, such as books, websites and articles, for improving your skil
 
 * [Ops School](http://www.opsschool.org) - Comprehensive program that will help you learn to be an operations engineer.
 * [Digital Ocean Tutorials](https://www.digitalocean.com/community/tutorials) - A surprisingly vast resource for getting the basics of certain applications, tools, or even systems administration topics.
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+## License
+
+![CC BY 4.0](http://i.creativecommons.org/l/by/4.0/88x31.png)
+
+This work is licensed under a Creative Commons Attribution 4.0 International license.