Update README.txt for Ranger

Author: michaelgelliot

ranger/README.txt

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+Fuzzy Droplets includes a lightly modified subsection of Ranger.
+Please see the following url for further information: https://github.com/imbs-hl/ranger
+
+Citation: 
+
+Wright, M. N. & Ziegler, A. (2017). 
+Ranger: A fast implementation of random forests for high dimensional data in C++ and R. 
+J Stat Softw 77:1-17. 
+https://doi.org/10.18637/jss.v077.i01.