Add new permission on readme (#42)

Finally I hit spot instance request with `capacity-not-available`.
It work as expected. And need add permission for describe spot request.

README.md

@@ -28,6 +28,7 @@ The IAM role of the node that runs the minion-manager should have the following
         "ec2:DescribeInstances",
         "ec2:TerminateInstances",
         "ec2:DescribeSpotPriceHistory",
+        "ec2:DescribeSpotInstanceRequests",
         "autoscaling:CreateLaunchConfiguration",
         "autoscaling:DeleteLaunchConfiguration",
         "autoscaling:DescribeLaunchConfigurations",