Add link to CloudWatch Metric Datum

[pcolazurdo]

In the home page of the project, when referring to:

"Units must meet CW Metrics unit requirements, if not it will default to None."

it would be useful to have a link to: https://docs.aws.amazon.com/AmazonCloudWatch/latest/APIReference/API_MetricDatum.html so users can refer to the official documentation directly to avoid confusion.