Update Spot instances to reflect terminations (#723)

There are CW events for this now. Huzzah!

README.md

@@ -2395,7 +2395,7 @@ Billing and Cost Management
 	-	Prices are per instance type and per availability zone. The same instance type may have wildly different price in different zones at the same time. Different instance types can have very different prices, even for similarly powered instance types in the same zone.
 	-	Compare prices across instance types for better deals.
 	-	Use Spot instances whenever possible. Setting a high bid price will assure your machines stay up the vast majority of the time, at a fraction of the price of normal instances.
-	-	Get notified up to two minutes before price-triggered shutdown by polling [your Spot instances’ metadata](https://aws.amazon.com/blogs/aws/new-ec2-spot-instance-termination-notices/).
+	-	Get notified up to two minutes before price-triggered shutdown by polling [your Spot instances’ metadata](https://aws.amazon.com/blogs/aws/new-ec2-spot-instance-termination-notices/), or by watching for [the termination CloudWatch event](https://aws.amazon.com/about-aws/whats-new/2018/01/amazon-ec2-spot-two-minute-warning-is-now-available-via-amazon-cloudwatch-events/).
 	-	Make sure your usage profile works well for Spot before investing heavily in tools to manage a particular configuration.
 -	**Spot fleet:**
 	-	You can realize even bigger cost reductions at the same time as improvements to fleet stability relative to regular Spot usage by using [Spot fleet](http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/spot-fleet.html) to bid on instances across instance types, availability zones, and (through multiple Spot Fleet Requests) regions.