Temporary doc draft for openems usage.

For ports, place padstacks with either top or bottom copper shape and _no_
hole. Then set the attribute openems::vport on the padstack. The value
should be the name of the port as to be seen in openems.

List of openems-related attributes:

openems::vport          - mandatory: port name
openems::resistance     - optional: overrides default port resistance
openems::active         - optional: either true or false; default=true
openems::vport-reverse  - optional: when present, swap start/end