DOC: Add explanation for dividing by 2
DOC: Add explanation for dividing by 2

Inline comment about computing the average of the buys and sells.

femtotrader/pyfolio,YihaoLu/pyfolio,YihaoLu/pyfolio,dursk/pyfolio,r0k3/pyfolio,ChinaQuants/pyfolio,chayapan/pyfolio,r0k3/pyfolio,dursk/pyfolio,femtotrader/pyfolio,raybuhr/pyfolio,yanheven/pyfolio,quantopian/pyfolio,sketchytechky/pyfolio,yanheven/pyfolio,sketchytechky/pyfolio,ChinaQuants/pyfolio,quantopian/pyfolio,raybuhr/pyfolio