Add tutorial for distributed training and batch prediction with Kubernetes (#4621)

* provide the readme

* update for format

* reformat

* reformat -2

* update again

* update format

* update w.r.t yinlou's comments

* Add kubernetes tutorial to Table of Contents

* Style edit

doc/tutorials/index.rst

@@ -11,6 +11,7 @@ See `Awesome XGBoost <https://github.com/dmlc/xgboost/tree/master/demo>`_ for mo
 
   model
   Distributed XGBoost with AWS YARN <aws_yarn>
+  kubernetes
   Distributed XGBoost with XGBoost4J-Spark <https://xgboost.readthedocs.io/en/latest/jvm/xgboost4j_spark_tutorial.html>
   dart
   monotonic