Add Deprecation Note in README.md

DEPRECATED: Preliminary dataset frozen — migrate to TroveLedger

This dataset has officially left its preliminary status and will no longer receive updates.

All accumulated data has been transferred to the new dataset:
Traders-Lab/TroveLedger

TroveLedger continues the same directory structure and data format,
so no changes to existing code are required.

All future updates and newly added indices will happen exclusively there.

README.md

@@ -5,9 +5,23 @@ license_link: LICENSE
 tags:
 - finance
 ---
-# Preliminary Financial Time Series Dataset (Version 2)
+> ## ⚠️ Deprecation Notice
+>
+> This dataset has officially left its **preliminary** phase and is now **deprecated**.
+> It will no longer receive updates or new data.
+>
+> All accumulated historical data has been transferred to the new dataset:
+>
+> ➡️ **[Traders-Lab / TroveLedger](/datasets/Traders-Lab/TroveLedger)**
+>
+> TroveLedger continues the same directory structure and data format,
+>so no changes to existing code are required.
+>
+> All future updates, newly added indices, and ongoing data accumulation
+> will happen **exclusively** in TroveLedger.
+>
+> This dataset remains available temporarily for archival and transition purposes.
 
-⚠️ Warning: This is a second preliminary dataset for development, testing, and feedback purposes. A stable, production-ready dataset will be released later. This time, all data in here will be available in future "stable" datasets (I guesss.. )
 
 ## Overview