docs(synthetics): canary schedule section in README (aws#15352)

This PR adds more extensive documentation for how to schedule CloudWatch Synthetics canary cron jobs via CDK, because today the service documentation for how to schedule cron jobs is lacking and takes developers a lot of investigation to figure out how to make it work successfully.

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-synthetics/README.md

@@ -87,6 +87,27 @@ The Canary code will be executed in a lambda function created by Synthetics on y
 
 To learn more about Synthetics capabilities, check out the [docs](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/CloudWatch_Synthetics_Canaries.html).
 
+
+### Canary Schedule
+
+You can specify the schedule on which a canary runs by providing a
+[`Schedule`](https://docs.aws.amazon.com/cdk/api/latest/docs/@aws-cdk_aws-synthetics.Schedule.html)
+object to the `schedule` property.
+
+Configure a run rate of up to 60 minutes with `Schedule.rate`:
+
+```ts
+Schedule.rate(Duration.minutes(5)), // Runs every 5 minutes.
+```
+
+You can also specify a [cron expression](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/CloudWatch_Synthetics_Canaries_cron.html) via `Schedule.expression`:
+
+```ts
+Schedule.expression('cron(0 0,8,16 * * ? *)'), // Run at 12am, 8am, 4pm UTC every day
+```
+
+If you want the canary to run just once upon deployment, you can use `Schedule.once()`.
+
 ### Configuring the Canary Script
 
 To configure the script the canary executes, use the `test` property. The `test` property accepts a `Test` instance that can be initialized by the `Test` class static methods. Currently, the only implemented method is `Test.custom()`, which allows you to bring your own code. In the future, other methods will be added. `Test.custom()` accepts `code` and `handler` properties -- both are required by Synthetics to create a lambda function on your behalf.