Update README.md to add Postgres 16

Author: Morl99

README.md

@@ -10,6 +10,7 @@ It pushes a nightly latest image to ghcr.io
 docker run ghcr.io/dbsystel/postgresql-partman:13
 docker run ghcr.io/dbsystel/postgresql-partman:14
 docker run ghcr.io/dbsystel/postgresql-partman:15
+docker run ghcr.io/dbsystel/postgresql-partman:16
 ```
 
 The tags represent the postgresql major version. They will contain the latest available pg_partman version that was available at build time.