Add instructions on how to "open loop" about docs HOT 4 CLOSED

(Add to the docs here: http://openaps.readthedocs.io/en/latest/docs/Customize-Iterate/usability-considerations.html#how-do-i-open-loop), please open additional doc PRs if you would like to add this info, or a link to it elsewhere.

from docs.

@danamlewis - you can assign this to me, but I would like a recommendation for "where" to add this.

from docs.

Any update @endafarrell ? I'm having a difficult time finding and understanding the "open loop" setup instructions on the latest docs.

from docs.

@MrJarnould the easiest way to open loop is to set your basal type to be % - that blocks the rigs from setting temporary basal rates on the pump, but allows you to collect info from the pump and you can view the recommendations in NS or in the logs on the rig to see what it would otherwise be doing.

from docs.