add license info

Author: mine-cetinkaya-rundel

README.md

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 # openintro-statistics-slides
-Slides for the OpenIntro Statistics textbook
+
+Slides for the OpenIntro Statistics textbook, Second Edition.
+
+These slides are available at http://www.openintro.org under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported license (CC BY-NC-SA):
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+http://creativecommons.org/licenses/by-nc-sa/3.0/
+
+This file describes guidelines for when the slides' source files are modified and/or shared. The CC BY-SA license guidelines supercede any guidelines put forth in this file; follow the CC BY-SA license if there is any discrepancy between that license and these guidelines.
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+1. Communication obligation. Any derivative work must communicate that it is licensed under a CC BY-SA license, and it also must in some way include the attribution content contained in the footnote on the first page of the original document.
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+2. Derivative title. No derivative may include "OpenIntro" in the title, unless it is included in text of the form "Derivative of OpenIntro", e.g. one might add a subtitle such as "Derivative of OpenIntro Slides".
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+3. For derivative works, we suggest but do not require that contributing authors' names be listed in chronological order of their contribution.