added license back in after Talamo deleted it.

license.md

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+This work is licensed under the Creative Commons
+Attribution-NonCommercial-ShareAlike License. To view a copy of this
+license, visit
+
+  http://creativecommons.org/licenses/by-nc-sa/4.0/
+
+or send a letter to Creative Commons, 543 Howard Street, 5th Floor,
+San Francisco, California, 94105, USA. If you distribute this work or
+a derivative, include the history of the document.
+
+The source code is available at
+
+  https://github.com/mooculus/calculus
+
+This text contains source material from several other open-source
+texts:
+
+* Single and Multivariable Calculus: Early
+  Transcendentals. Guichard. Copyright 2015 Guichard, Creative Commons
+  Attribution-NonCommercial-ShareAlike License 3.0.
+
+	http://communitycalculus.org/
+
+* APEX Calculus. Hartman, Heinold, Siemers, Chalishajar, Bowen
+  (Ed.). Copyright 2014 Hartman, Creative Commons
+  Attribution-Noncommercial 3.0.
+
+	http://www.apexcalculus.com/
+
+* Elementary Calculus: An Infinitesimal Approach. Keisler. Copyright
+  2015 Keisler, Creative Commons Attribution-NonCommercial-ShareAlike
+  License 3.0.
+
+	http://www.math.wisc.edu/~keisler/calc.html
+
+We will be glad to receive corrections and suggestions for improvement
+at ximera@math.osu.edu