Clarify upstream support

This commit clarifies that upstream is not involved with the Docker Hub
image. This is an important clarification as the Docker Hub definition
states:

   This team works in collaboration with upstream software maintainers,
   security experts, and the broader Docker community.

For this image this is not correct. As this causes confusion within the
Docker and Elasticsearch communities, it's important to clarify these
remarks.

Author: jasontedor

README.md

@@ -1,6 +1,6 @@
 # About this Repo
 
-This is the Git repo of the Docker [official image](https://docs.docker.com/docker-hub/official_repos/) for [elasticsearch](https://registry.hub.docker.com/_/elasticsearch/). See [the Docker Hub page](https://registry.hub.docker.com/_/elasticsearch/) for the full readme on how to use this Docker image and for information regarding contributing and issues.
+This is the Git repo of the Docker Hub [image](https://docs.docker.com/docker-hub/official_repos/) for [Elasticsearch](https://registry.hub.docker.com/_/elasticsearch/). See [the Docker Hub page](https://registry.hub.docker.com/_/elasticsearch/) for the full readme on how to use this Docker image and for information regarding contributing and issues. The Docker Hub image is not prepared in collaboration with nor supported by upstream [Elastic](https://elastic.co) which provides its [own official image](https://www.elastic.co/guide/en/elasticsearch/reference/current/docker.html).
 
 The full readme is generated over in [docker-library/docs](https://github.com/docker-library/docs), specifically in [docker-library/docs/elasticsearch](https://github.com/docker-library/docs/tree/master/elasticsearch).