Add support for upgrading Postgres

Since cbee084ac1, this playbook supports Postgres 10.x,
but keeps existing Postgres-9.x installs on 9.x.

This playbook can now also be ran with `--tags=upgrade-postgres`
to make it upgrade from Postgres 9.x to 10.x (or other versions
in the future).

README.md

@@ -259,6 +259,28 @@ You can do it via this Ansible playbook (make sure to edit the `<your-username>`
 **Note**: `<your-username>` is just a plain username (like `john`), not your full `@<username>:<your-domain>` identifier.
 
 
+### Upgrading Postgres
+
+If you're not using an external Postgres server, this playbook initially installs Postgres for you.
+
+Once installed like that, this playbook attempts to preserve the Postgres version it starts with.
+This is because newer Postgres versions cannot start with data generated by an older Postgres version.
+An upgrade must be performed.
+
+This playbook can upgrade your existing Postgres setup with the following command:
+
+	ansible-playbook -i inventory/hosts setup.yml --tags=upgrade-postgres
+
+**The old Postgres data directory is backed up** (by renaming to `/matrix/postgres-auto-upgrade-backup`).
+It stays around forever, until you **manually decide to delete it**.
+
+As part of the upgrade, the database is dumped to `/tmp`, upgraded and then restored from that dump.
+To use a different directory, pass some extra flags to the command above, like this: `--extra-vars="postgres_dump_dir=/directory/to/dump/here"`
+
+**ONLY one database is migrated** (the one specified in `matrix_postgres_db_name`, named `homeserver` by default).
+If you've created other databases in that database instance (something this playbook never does and never advises), data will be lost.
+
+
 ## Uninstalling
 
 **Note**: If you have some trouble with your installation configuration, you can just re-run the playbook and it will try to set things up again. You don't need to uninstall and install fresh.