Update README.md

README.md

-For more information about theC(canonical) Scan Matcher, see the webpage: http://purl.org/censi/2007/csm.
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+For more information about the C(anonical) Scan Matcher, see the webpage: http://purl.org/censi/2007/csm .
+
+This is the "master" branch of CSM, which uses GSL.
+
+There is also another branch, called [``csm_eigen``][branch], which uses the ``eigen`` library. 
+This branch is the work of people working at U. Freiburg and Kuka, including
+Christoph Sprunk and Rainer Kuemmerle.
+
+[branch]: https://github.com/AndreaCensi/csm/tree/csm_eigen