DOC: Clarify OHLCV_AGG docstring code example

Author: kernc

backtesting/lib.py

@@ -37,7 +37,7 @@
 """Dictionary of rules for aggregating resampled OHLCV data frames,
 e.g.
 
-    df.resample('4H', label='right').agg(OHLCV_AGG)
+    df.resample('4H', label='right').agg(OHLCV_AGG).dropna()
 """
 
 TRADES_AGG = OrderedDict((