Announce Postgres v13 upgrade

CHANGELOG.md

@@ -1,3 +1,11 @@
+# 2020-10-01
+
+## Postgres 13 support
+
+The playbook now installs [Postgres 13](https://www.postgresql.org/about/news/postgresql-13-released-2077/) by default.
+
+If you have have an existing setup, it's likely running on an older Postgres version (9.x, 10.x, 11.x or 12.x). You can easily upgrade by following the [upgrading PostgreSQL guide](docs/maintenance-postgres.md#upgrading-postgresql).
+
 # 2020-09-01
 
 ## matrix-registration support