Add CC BY-NC-SA 4.0 license

README.md

@@ -12,6 +12,10 @@ _Source: Yubico_
 
 This book is [available online for free](https://ruimarinho.gitbooks.io/yubikey-handbook/content/) and is downloadable in [PDF](https://www.gitbook.com/download/pdf/book/ruimarinho/yubikey-handbook), [ePUB](https://www.gitbook.com/download/epub/book/ruimarinho/yubikey-handbook) or [Mobi/Kindle](https://www.gitbook.com/download/mobi/book/ruimarinho/yubikey-handbook) formats.
 
+# License
+
+<a rel="license" href="http://creativecommons.org/licenses/by-nc-sa/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by-nc-sa/4.0/88x31.png" /></a><br />This work is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-nc-sa/4.0/">Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License</a>.
+
 # About the author
 
 Rui Marinho ([github](https://github.com/ruimarinho), [twitter](https://twitter.com/ruipmarinho), [npm](https://www.npmjs.com/~ruimarinho)) is a software engineer by day, a security engineer by night and a network engineer on weekends. This blend of interests has allowed him to plan and build large-scale infrastructure projects, having specialized in mission-critical and highly available systems with security as one of its top priorities. [Read more](introduction/about-the-author.md).