add release note v1.21.0

doc/release/v1.21.0.md

@@ -0,0 +1,23 @@
+
+## Fixes
+
+- Fixed back-test concurrent read/write map issue
+- Fixed back-test go routine initialization
+- Fixed trade notification format - remove trade trailing zero digits
+- Fixed dockerfile
+- Fixed back-test kline sync issue
+
+## Refactoring
+
+- Refactored and fixed depth buffer
+
+## Strategy
+
+- Added rebalance strategy
+
+## Features
+
+- Added prometheus metrics support
+- Added kucoin exchange support
+- Added bookTicker support to binance, okex, ftx
+- Integrated net asset value recording table in db