in usage, link to artifact and import Unit

It was a little difficult to find the actual artifact location, so I figured it might be helpful to include that in the README.

As a drive-by, I've included the import for the `Unit` class in the usage example.

Author: yegeniy

README.md

@@ -28,6 +28,7 @@ To use a metric logger, you need to manually create and flush the logger.
 ```java
 import software.amazon.cloudwatchlogs.emf.logger.MetricsLogger;
 import software.amazon.cloudwatchlogs.emf.model.DimensionSet;
+import software.amazon.cloudwatchlogs.emf.model.Unit;
 
 class Example {
 	public static void main(String[] args) {
@@ -40,6 +41,10 @@ class Example {
 }
 ```
 
+You can find the artifact location and examples of how to include it in your project at [Maven Central](https://search.maven.org/artifact/software.amazon.cloudwatchlogs/aws-embedded-metrics):
+
+https://search.maven.org/artifact/software.amazon.cloudwatchlogs/aws-embedded-metrics
+
 ## API
 
 ### MetricsLogger