Mention Postgres + SSL in the docs

Refers to #89 (Github Issue)

docs/configuring-playbook-external-postgres.md

@@ -19,4 +19,6 @@ matrix_synapse_database_database: "your-postgres-server-database-name"
 The database (as specified in `matrix_synapse_database_database`) must exist and be accessible with the given credentials.
 It must be empty or contain a valid Synapse database. If empty, Synapse would populate it the first time it runs.
 
-**Note**: the external server that you specify in `matrix_synapse_database_host` must be accessible from within the `matrix-synapse` Docker container (and possibly other containers too). This means that it either needs to be a publicly accessible hostname or that it's a hostname on the same Docker network where all containers installed by this playbook run (a network called `matrix` by default). Using a local PostgreSQL instance on the host (running on the same machine, but not in a container) is not possible.
+**Note**: the external server that you specify in `matrix_synapse_database_host` must be accessible from within the `matrix-synapse` Docker container (and possibly other containers too). This means that it either needs to be a publicly accessible hostname or that it's a hostname on the same Docker network where all containers installed by this playbook run (a network called `matrix` by default). Using a local PostgreSQL instance on the host (running on the same machine, but not in a container) is not possible.
+
+The connection to your external Postgres server **will not be SSL encrypted**, as [we don't support that yet](https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/89).