refine doc, with Rd

R-package/vignettes/xgboost.bib

@@ -18,3 +18,13 @@
   publisher={Institute of Mathematical Statistics}
 }
 
+
+@misc{
+    Bache+Lichman:2013 ,
+    author = "K. Bache and M. Lichman",
+    year = "2013",
+    title = "{UCI} Machine Learning Repository",
+    url = "http://archive.ics.uci.edu/ml",
+    institution = "University of California, Irvine, School of Information and Computer Sciences" 
+}
+