[docs] Add example showing how to use Ray on Kubernetes. (#3126)

Closes #1353.

kubernetes/example.py

@@ -0,0 +1,38 @@
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+from collections import Counter
+import socket
+import sys
+import time
+import ray
+
+if __name__ == "__main__":
+    ray.init(redis_address="{}:6379".format(socket.gethostbyname("ray-head")))
+
+    # Wait for all 4 nodes to join the cluster.
+    while True:
+        num_nodes = len(ray.global_state.client_table())
+        if num_nodes < 4:
+            print("{} nodes have joined so far. Waiting for more."
+                  .format(num_nodes))
+            sys.stdout.flush()
+            time.sleep(1)
+        else:
+            break
+
+    @ray.remote
+    def f(x):
+        time.sleep(0.01)
+        return x + (ray.services.get_node_ip_address(), )
+
+    # Check that objects can be transferred from each node to each other node.
+    for i in range(100):
+        print("Iteration {}".format(i))
+        sys.stdout.flush()
+        print(Counter(ray.get([f.remote(f.remote(())) for _ in range(10000)])))
+        sys.stdout.flush()
+
+    print("Success!")
+    sys.stdout.flush()