Adding review of Bittorrent Economics paper (#81)

* Adding review of Bittorrent Economics paper

* Clarify point about old files credits in papers/Economics_of_BitTorrent_communities/README.md

Co-authored-by: Dmitriy Ryajov <dryajov@gmail.com>

Co-authored-by: Dmitriy Ryajov <dryajov@gmail.com>

papers/Economics_of_BitTorrent_communities/README.md

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+# Economics of BitTorrent communities
+
+## Authors
+
+- Ian A. Kash - iankash@microsoft.com
+- John K. Lai - jklai@seas.harvard.edu
+- Haoqi Zhang - hq@eecs.harvard.edu
+- Aviv Zohar - avivz@microsoft.com
+
+### DOI
+
+- https://doi.org/10.1145/2187836.2187867
+
+## Summary
+
+The paper is a study of a BitTorrent community called DIME, where users share live concert recordings. The community has around 100K users and the study analyses data gathered over 6 months.
+
+### Main ideas
+
+ * The DIME system enforces a ratio of at least 0.25: 4 downloads for 1 upload
+ * Many users have a ratio above 1 (which shows an altruistic behaviour)
+ * New files are more attractive to users and have high demand at the beginning
+ * Users with high bandwidth Internet connections take advantage of new files to take credits
+ * Old files are no good to gain credit because they are not in high demand
+ * There are periods where downloads are free
+ * Users prefer to download old files during free periods
+
+### Observations
+
+ * The paper does not give any numbers about the amount of data available in total
+ * The paper does not provide data about the file size distribution
+ * Overall the paper provides interesting data about how sharing communities behave but no data about the decentralized storage itself.
+ 
+### Other ideas
+
+ * Some aspects of the demand for files with respect to their life could be applied to other decentralized storage systems 
+
+