Add readme for postgresql support

uncloud_django_based/uncloud/doc/README-postgresql.org

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+* uncloud clients access the data base from a variety of outside hosts
+* So the postgresql data base needs to be remotely accessible
+* Instead of exposing the tcp socket, we make postgresql bind to localhost via IPv6
+** ::1, port 5432
+* Then we remotely connect to the database server with ssh tunneling
+** ssh -L5432:localhost:5432 uncloud-database-host
+* Configuring your database for SSH based remote access
+** host    all             all             ::1/128                 trust