Update README.md

README.md

@@ -7,3 +7,6 @@ This book uses the modeling software NetLogo. If you do not have NetLogo v6 alre
 <a href="https://ccl.northwestern.edu/netlogo/download.shtml">Download NetLogo</a>.
 
 Note that on GitHub the usual method of right-clicking a file and choosing "Save Link As..." will not work (you'll get a github webpage version of the file which won't work in NetLogo). Instead left-click on the file you want, then locate the `RAW` button in the top-right. Right-click that and "Save Link As..." to download the files correctly. Alternatively, install [GitHub Desktop](https://desktop.github.com/) and *clone* the whole repository (~100mb).
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+This textbook and code repository are licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.