Documentation: Adding an alarm to watchful's scope

[eisenhowerj]

Really enjoying cdk-watchful but I wanted to add my own custom alarm, here is how I did it:

        watchful.watch_lambda_function("MyLambda", my_lambda)
        lambda_invocations = my_lambda.metric_invocations(period=cdk.Duration.minutes(1))
        invocation_alarm = cloudwatch.Alarm(
            self,
            "Invocations",
            metric=lambda_invocations,
            evaluation_periods=1,
            threshold=20,
        )
        watchful.add_alarm(invocation_alarm)