docs(readme): add output format, CSV columns, rate-limit and visibility notes

README.md

@@ -1,6 +1,6 @@
 # export-telegram-group-members
 
-Export Telegram group members
+Export Telegram group members to CSV using Telethon — admin auth required for full member visibility.
 
 ## How to use
 
@@ -23,3 +23,25 @@ pip install -r requirements.txt
 ```bash
 python main.py
 ```
+
+## Output format
+
+Results are saved to a timestamped folder (e.g. `2025-01-15 10-30-00/`). Each group produces one CSV file named after the group title:
+
+```
+id,username,first_name,last_name
+123456789,johndoe,John,Doe
+987654321,,Jane,Smith
+```
+
+Columns: `id`, `username`, `first_name`, `last_name`. Members without a username have an empty `username` field.
+
+## Rate limits and visibility notes
+
+- Telegram limits how fast you can fetch participants; large groups may take longer.
+- For **supergroups**, only admins can retrieve the full member list — regular members see a partial list or get an error.
+- For private groups where you are not a member, access will be denied.
+
+## License
+
+Apache-2.0 — see [LICENSE](LICENSE).