Update README.md

added license info

README.md

@@ -11,3 +11,8 @@ Mastering Bitcoin is a book for developers, but the first two chapters cover bit
 # Source 
 
 The book's source code, found in this repository, is a near-final draft before the publisher's editorial process. Please do not submit changes here, instead use the [errata](http://www.oreilly.com/catalog/errata.csp?isbn=0636920032281) page on the publisher's site. 
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+<a rel="license" href="http://creativecommons.org/licenses/by-sa/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by-sa/4.0/88x31.png" /></a><br /><span xmlns:dct="http://purl.org/dc/terms/" href="http://purl.org/dc/dcmitype/Text" property="dct:title" rel="dct:type">Mastering Bitcoin</span> by <a xmlns:cc="http://creativecommons.org/ns#" href="http://antonopoulos.com/" property="cc:attributionName" rel="cc:attributionURL">Andreas M. Antonopoulos</a> is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-sa/4.0/">Creative Commons Attribution-ShareAlike 4.0 International License</a>.