Add a CHANGELOG file for release notes. (elastic#29450)

* Add a CHANGELOG file for 7.x release notes.

* update file to include 6.x

* remove confusing comment and small edit to section title

* moving CHANGELOG file under docs directory, as it pertains to release notes.

Author: Paul Sanwald

.gitattributes

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+CHANGELOG.asciidoc  merge=union

docs/CHANGELOG.asciidoc

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+// Use these for links to issue and pulls. Note issues and pulls redirect one to
+// each other on Github, so don't worry too much on using the right prefix.
+// :issue: https://github.com/elastic/elasticsearch/issues/
+// :pull: https://github.com/elastic/elasticsearch/pull/
+
+= Elasticsearch Release Notes
+
+== Elasticsearch 7.0.0
+
+=== Breaking Changes
+
+=== Breaking Java Changes
+
+=== Deprecations
+
+=== New Features 
+
+=== Enhancements
+
+=== Bug Fixes
+
+=== Regressions
+
+=== Known Issues
+
+== Elasticsearch version 6.3.0
+
+=== New Features 
+
+=== Enhancements
+
+=== Bug Fixes
+
+=== Regressions
+
+=== Known Issues
+
+