README.md: more examples about ma

Author: kaelzhang

docs/README.md

@@ -498,6 +498,9 @@ Advanced usage
 ```py
 # The 5-period moving average for the upper bollinger band
 stock['ma:5,(boll.upper:21,2)']
+
+# The change rate of the series above ↑
+stock['change:(ma:5,(boll.upper:21,2))']
 ```
 
 ### `ema`, Exponential Moving Average

test/test_basic.py

@@ -106,6 +106,8 @@ def test_ma(stock):
 
     assert mama2.iloc[-1] == (ma2[-1] + ma2[-2]) / 2
 
+    stock['change:(ma:5,(boll.upper:21,2))']
+
 
 COMMANDS = [
     'ma:{}',