Add rate-limiting note for Kraken datadownload

docs/exchanges.md

@@ -30,6 +30,15 @@ Binance has been split into 3, and users must use the correct ccxt exchange ID f
 The Kraken API does only provide 720 historic candles, which is sufficient for Freqtrade dry-run and live trade modes, but is a problem for backtesting.
 To download data for the Kraken exchange, using `--dl-trades` is mandatory, otherwise the bot will download the same 720 candles over and over, and you'll not have enough backtest data.
 
+Due to the heavy rate-limiting applied by Kraken, the follwoing configuration section should be used to download data:
+
+``` json
+    "ccxt_async_config": {
+        "enableRateLimit": true,
+        "rateLimit": 3100
+    },
+```
+
 ## Bittrex
 
 ### Order types