Updated Readme.md to mention minimum Bottlerocket version requirement

During bugbash some developers started using Bottlerocket AMI version
less than 1.0.6. Due to this there were some errors in log but it was
not clear what was wrong. This change updates the Readme docs to mention
the minimum Bottlerocket version requirement for ECS Updater service to
work.

Author: srgothi92

README.md

@@ -61,6 +61,10 @@ A container instance may be skipped for update when:
   If draining has not completed by the end of the period, the updater will restore the instance and move to the next one.
   The time it takes for a task to be stopped is related to the `stopTimeout` task definition parameter and to any associated resources like load balancers.
   If your tasks are taking too long to drain, you can ensure that your task responds to `SIGTERM`, shorten the `stopTimeout`, or shorten the load balancer's health check and deregistration delay settings.
+* _Bottlerocket version is too old._
+  The Bottlerocket ECS Updater uses newer [`apiclient update` commands](https://github.com/bottlerocket-os/bottlerocket#update-api) that were added in version [1.0.5](https://github.com/bottlerocket-os/bottlerocket/blob/develop/CHANGELOG.md#v105-2021-01-15).
+  The SSM commands will fail if your Bottlerocket OS version is less than 1.0.5.
+  Instances running Bottlerocket versions less than 1.0.5 need to be manually updated.
 
 ### Why do new container instances launch with older Bottlerocket versions?