MAGEDOC-3150: Add link to release notes in magento2 CHANGELOG.md

- Release notes for 2.3.0 added

Author: jcalcaben

CHANGELOG.md

@@ -1,3 +1,7 @@
+2.3.0
+=============
+To get detailed information about changes in Magento 2.3.0, see the [Release Notes](https://devdocs.magento.com/guides/v2.3/release-notes/bk-release-notes.html)
+
 2.1.0
 =============
 To get detailed information about changes in Magento 2.1.0, please visit [Magento Community Edition (CE) Release Notes](http://devdocs.magento.com/guides/v2.1/release-notes/ReleaseNotes2.1.0CE.html "Magento Community Edition (CE) Release Notes")