docs: update README with CSV replace mode and initial data loading

README.md

@@ -17,6 +17,8 @@ Candle Annotator provides a TradingView-like charting interface that allows trad
 
 ### Data Management
 - **CSV Upload**: Import OHLC data with support for both Unix timestamps and date strings
+  - **Replace Mode**: Uploading a new CSV deletes all old candles and replaces them with new data
+  - **Initial Data**: Docker containers automatically load EURUSD.csv on first startup if database is empty
 - **SQLite Storage**: All candle data and annotations stored locally in SQLite database
 - **Data Persistence**: Annotations and candles persist between sessions
 
@@ -183,6 +185,7 @@ timestamp,label_type,price
 ### POST /api/upload
 Upload CSV file and store candle data
 
+**Behavior**: Deletes all existing candles before inserting new data (replace mode)  
 **Request**: multipart/form-data with `file` field  
 **Response**: `{ success: true, count: number }` or `{ error: string }`