Add warning about postgres password for migration

While migrating Gogs.

Guides/gogs.md

@@ -80,6 +80,8 @@ Gogs natively manages backups. Prefer to use gogs tools instead of _squared whee
 
 `$ ./gogs restore --help`
 
+**Beware** : please ensure that the user `gogs_app` has the same password on both instances ! You can change the password after the migration by using `ALTER ROLE gogs_app ENCRYPTED PASSWORD 'NewPassword';` and by changing the db password in `gogs/custom/conf/app.ini`.
+
 ### Repositories
 
 `/home/gituser/gogs/gogs-repositories/`