change location and template of vignette

R-package/vignettes/xgboost.bib

@@ -0,0 +1,20 @@
+@article{friedman2001greedy,
+    title={Greedy function approximation: a gradient boosting machine},
+    author={Friedman, Jerome H},
+    journal={Annals of Statistics},
+    pages={1189--1232},
+    year={2001},
+    publisher={JSTOR}
+}
+
+@article{friedman2000additive,
+  title={Additive logistic regression: a statistical view of boosting (with discussion and a rejoinder by the authors)},
+  author={Friedman, Jerome and Hastie, Trevor and Tibshirani, Robert and others},
+  journal={The annals of statistics},
+  volume={28},
+  number={2},
+  pages={337--407},
+  year={2000},
+  publisher={Institute of Mathematical Statistics}
+}
+