doc: add persistence section

doc/topics/developing-strategy.md

@@ -457,9 +457,47 @@ bbgo.OnShutdown(func(ctx context.Context, wg *sync.WaitGroup) {
 
 ## Persistence
 
+When you need to adjust the parameters and restart BBGO process, everything in the memory will be reset after the
+restart, how can we keep these data?
 
+Although BBGO is written in Golang, BBGO provides a useful dynamic system to help you persist your data.
 
+If you have some state needs to preserve before shutting down, you can simply add the `persistence` struct tag to the field,
+and BBGO will automatically save and restore your state. For example,
 
+```go
+type Strategy struct {
+	Position    *types.Position    `persistence:"position"`
+	ProfitStats *types.ProfitStats `persistence:"profit_stats"`
+	TradeStats  *types.TradeStats  `persistence:"trade_stats"`
+}
+```
 
+And remember to add the `persistence` section in your bbgo.yaml config:
 
+```yaml
+persistence:
+  redis:
+    host: 127.0.0.1
+    port: 6379
+    db: 0
+```
+
+In the Run method of your strategy, you need to check if these fields are nil, and you need to initialize them:
+
+```go
+	if s.Position == nil {
+		s.Position = types.NewPositionFromMarket(s.Market)
+	}
+
+	if s.ProfitStats == nil {
+		s.ProfitStats = types.NewProfitStats(s.Market)
+	}
+
+	if s.TradeStats == nil {
+		s.TradeStats = types.NewTradeStats(s.Symbol)
+	}
+```
+
+That's it. Hit Ctrl-C and you should see BBGO saving your strategy states.