add package credits.

README.txt

@@ -13,6 +13,22 @@ Look at R code to see how it works.
 Requirements
 ------------
 
-ArcGIS R bridge
-R Statistical Computing Software
-Geostatistical Analyst Extension for ArcGIS
+ - [ArcGIS R bridge](https://github.com/R-ArcGIS/r-bridge-install)
+ - [R Statistical Computing Software](http://www.r-project.org)
+
+###optional
+
+ - Geostatistical Analyst Extension for ArcGIS
+
+## Credits
+
+Model Based Clustering uses the [mclust package](http://www.stat.washington.edu/mclust/) for R:
+
+> `mclust`: Normal Mixture Modeling for Model-Based Clustering, Classification, and Density Estimation
+> By Chris Fraley, Adrian E. Raftery, T. Brendan Murphy, and Luca Scrucca
+> Licensed under the GPLv2
+
+All tools depend on the R Statistical Computing Software:
+
+> Copyright (C) 2015 The R Foundation for Statistical Computing
+> R is free software and comes with ABSOLUTELY NO WARRANTY.