BERLIN (Reuters) - German energy firm E.ON on Thursday said it would invest $500 million (413.5 million pounds) in building a 440-megawatt onshore wind farm in Texas, adding it had already sold the subscription rights for 200 megawatts of the plant for 12 years.The construction of the plant in Willacy, Cameron and Hidalgo counties will start at the end of the year, it said.