Enable SSL for connections to database

Since the database can be exposed to the outer world, it seems better to
have an encrypted connection to it.
We automatically generate a self-signed certificate since it's only used
to encrypt the connection but one can easily replace it by a custom one,
just overwrite `server.crt` and `server.key` in the
/var/lib/dokku/services/postgres/DB_NAME/data directory.

README.md

@@ -156,3 +156,12 @@ The `import` command should be used with any non-plain-text files exported by `p
 ```shell
 $ dokku postgres:connect db < ./dump.sql
 ```
+
+## security
+
+The connection to the database is done over SSL. A self-signed certificate is
+automatically generated when creating the service.  It can be replaced by a
+custom certificate by overwriting the `server.crt` and `server.key` files in
+`/var/lib/dokku/services/postgres/<DB_NAME>/data`.
+The `server.key` must be chmoded to 600 and must be owned by the postgres user
+or root.