Add plugin-based host entry to CHANGELOG.md (#1710)

* Add plugin-based host entry to CHANGELOG.md

* Amend to de-emphasize domain language

This should hopefully make it more obvious that the plugin system is
currently internal and one cannot yet write 3rd party plugins for
Boundary.

CHANGELOG.md

@@ -11,6 +11,11 @@ Canonical reference for changes, improvements, and bugfixes for Boundary.
 
 ### New and Improved
 
+* Boundary now supports dynamic discovery of host resources using our (currently
+  internal) new plugin system. See the
+  [documentation](https://www.boundaryproject.io/docs) for configuration
+  instructions. Currently, only Azure and AWS are supported, but more providers
+  will be following in future releases.
 * workers: The existing worker connection replay prevention logic has been
   enhanced to be more robust against attackers that have decryption access to
   the shared `worker-auth` KMS key