Add license summary and full text, revise README.

sleipner/COPYING

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+SLEIPNER CO2 REFERENCE DATASET LICENSE
+
+THIS IS A HUMAN-READABLE SUMMARY OF AND NOT A SUBSTITUTE FOR THE LICENSE
+(link to full license provided below)
+
+You are free to:
+ - Download and use the Licensed Material for non-commercial and commercial purposes
+ - Create, produce and reproduce Adapted Material
+ - Share the Licensed Material and/or the Adapted Material
+
+Under the following terms:
+ - You may not sell the Licensed Material.
+ - You must give the Sleipner Group credit (see the Sleipner CO2
+   Reference Dataset License for Sleipner group members), and provide
+   a link to these terms and conditions, as well as a copyright notice
+   if applicable.
+ - You may not share Adapted Material under a license that prevents
+   recipients from complying with these terms and conditions.
+ - You shall not use the Licensed Material in a manner that appears
+   misleading nor present the Licensed Material in a distorted or
+   incorrect manner.
+
+This license is based on CC BY 4.0 license, two important changes are:
+ - The licensed material may not be sold
+ - The license covers all data in the dataset whether or not it is by law covered by copyright
+
+Link to full license text:
+
+  https://co2datashare.org/sleipner-2019-benchmark-model/static/license.pdf