docs: artifact type

Author: danielorihuela

docs/package_manager.md

@@ -62,6 +62,8 @@ Then we have the [manifest](https://github.com/opencontainers/image-spec/blob/ma
 
 In that file, Agent Control expects to find some specific keys and values.
 
+* `artifactType` must exist and have value `application/vnd.newrelic.agent.v1`
+
 * `layers/mediaType` must take one of the following values:
 
     - `application/vnd.newrelic.agent.content.v1.zip`