docs(#16): add link to units

Closes #16

Author: jaredcnance

README.md

@@ -62,7 +62,7 @@ Requirements:
 - Name Length 1-255 characters
 - Name must be ASCII characters only
 - Values must be in the range of 8.515920e-109 to 1.174271e+108. In addition, special values (for example, NaN, +Infinity, -Infinity) are not supported.
-- Units must meet CW Metrics unit requirements, if not it will default to None.
+- Units must meet CW Metrics unit requirements, if not it will default to None. See [MetricDatum](https://docs.aws.amazon.com/AmazonCloudWatch/latest/APIReference/API_MetricDatum.html) for valid values.
 
 Examples: