[Add] BitmexGateway: Added some comment to make logic more clear.
[Add] BitmexGateway: Added some comment to make logic more clear.

msincenselee/vnpy,vnpy/vnpy,msincenselee/vnpy,andrewchenshx/vnpy,bigdig/vnpy,bigdig/vnpy,andrewchenshx/vnpy,vnpy/vnpy,msincenselee/vnpy,bigdig/vnpy,bigdig/vnpy,andrewchenshx/vnpy,andrewchenshx/vnpy,msincenselee/vnpy,andrewchenshx/vnpy