Add changelog (#83830)

Having a `CHANGELOG.md` file in the root of a code repository is a
very common practice. Elasticsearch doesn't follow this for good
reasons, and publishes extensive release notes on the website.
However we can be more helpful to users browsing the repository by
pointing them to the release notes.

CHANGELOG.md

@@ -0,0 +1,3 @@
+# Elasticsearch Changlog
+
+Please see the [release notes](https://www.elastic.co/guide/en/elasticsearch/reference/current/es-release-notes.html) in the reference manual.