add license, closes #1

Author: cwickham

LICENSE.md

@@ -0,0 +1 @@
+<a rel="license" href="http://creativecommons.org/licenses/by-nc/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by-nc/4.0/80x15.png" /></a><br /><span xmlns:dct="http://purl.org/dc/terms/" property="dct:title">purrr tutorial</span> by <a xmlns:cc="http://creativecommons.org/ns#" href="https://github.com/cwickham/purrr-tutorial" property="cc:attributionName" rel="cc:attributionURL">Charlotte Wickham</a> is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-nc/4.0/">Creative Commons Attribution-NonCommercial 4.0 International License</a>.

README.md

@@ -11,3 +11,5 @@ Code with a lot of duplication is harder to understand, troubleshoot and maintai
 You'll learn to use the functions in the `purrr` package to perform iterative tasks: tasks that look like "for each _____ do _____".
 
 By the end of the tutorial you'll be writing code that is more readable and easier to update and you'll be ready to solve new iteration problems faster and with fewer mistakes.
+
+<a rel="license" href="http://creativecommons.org/licenses/by-nc/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by-nc/4.0/80x15.png" /></a><br />This <span xmlns:dct="http://purl.org/dc/terms/" property="dct:title">purrr tutorial</span> by <a xmlns:cc="http://creativecommons.org/ns#" href="https://github.com/cwickham/purrr-tutorial" property="cc:attributionName" rel="cc:attributionURL">Charlotte Wickham</a> is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-nc/4.0/">Creative Commons Attribution-NonCommercial 4.0 International License</a>.