What is the total capacity (MW) of the farm that is noted as being under construction?
CREATE TABLE "other_notable_onshore_wind_farms" (
  "wind_farm" text,
  "capacity_mw" real,
  "country" text,
  "state_province" text,
  "notes" text
);
SELECT COUNT("capacity_mw") FROM "other_notable_onshore_wind_farms" WHERE "notes"='under construction';