add documentation for migration command system (#652)

# Purpose
Added documentation and usage to the POSTGRESQL_SETUP.md file as well as
the migration script. This was for clarity and posterity purposes.

# New Changes
- Documentation

# Outstanding Changes
If there are non-critical changes (i.e. additional features) that can be
made to this feature in the future, indicate them here.

Add telemetry migration to the command system in the future

Author: Syzygicality

gs/POSTGRESQL_SETUP.md

@@ -67,6 +67,10 @@ Note: You can choose to remove the first to lines of the `.env` that start with
 
 Now, you can start the backend as by running `fastapi dev gs/backend/main.py` from the top level directory to run in development mode.
 
+To migrate pre-determined datainto your local database, you can run `python3 gs/backend/migrate.py` from the top level directory.
+
+Alternatively, you can include `callsigns` or `commands` as command arguments to migrate those respective datasets individually.
+
 **[Back to top](#table-of-contents)**
 
 ## Sources

gs/backend/migrate.py

@@ -3,6 +3,16 @@
 from gs.backend.data.database.engine import get_db_session
 from gs.backend.data.resources.utils import add_callsigns, add_main_commands
 
+"""
+To migrate pre-determined datainto your local database,
+you can run `python3 gs/backend/migrate.py` from the top
+level directory.
+
+Alternatively, you can include `callsigns` or `commands`
+as command arguments to migrate those respective datasets
+individually.
+"""
+
 if __name__ == "__main__":
     if len(sys.argv) > 2:
         raise ValueError(f"Invalid input. Expected at most 1 argument, received {len(sys.argv)}")