add rank regression example

Author: keepwith

doc/source/example.rst

@@ -11,6 +11,7 @@ Example Gallery
    examples/QR.ipynb
    examples/SVM.ipynb
    examples/FairSVM.ipynb
+   examples/RankRegression.ipynb
 
 List of Examples
 ----------------
@@ -20,4 +21,5 @@ List of Examples
 
    examples/QR.ipynb
    examples/SVM.ipynb
-   examples/FairSVM.ipynb
+   examples/FairSVM.ipynb
+   examples/RankRegression.ipynb