Beginning in the summer, The Washington Post will start charging frequent users of its website, asking those who look at more than 20 articles or multimedia features a month to pay a fee, although the company has not decided how much it will charge. The paper said, however, that it will exempt large parts of its audience from having to pay the fees. Its home-delivery subscribers will have free access to all of The Post’s digital products, and students, teachers, school administrators, government employees and military personnel will have unlimited access to the Web site while in their schools and workplaces.