It's our responsibility to bring change   For the last five years, Chadds Ford residents have depended upon the Chadds Ford Post to report cultural and political issues in our community. Recently, we got much more; we got an exposé.   When actions of election officials ran directly counter to the democratic process, the Post rose to meet its principle responsibility under the First Amendment. That is, the paper provided an outside check on political power. Editorials were sincere, explicit, unflattering -- in a word, truthful. Coverage of the Civic Association's covert balloting educated, aroused, and angered us.   And news of dishonesty in our election of civic officials gives rise to a second responsibility, one we all share, to do something about this. In the words of John F. Kennedy, "An error doesn't become a mistake until you refuse to correct it."   A first step would be the prompt nullification of the 47 unauthorized proxy ballots and provide Rob Porter his well-earned seat on the Chadds Ford Civic Association's Board of Directors. A second would be to call a town meeting to develop a nonpartisan process to ensure that Chadds Ford's civic elections are always democratic.   Chadds Ford's current political affairs are not good for anyone -- for our two parties, activists in various causes, or for our citizens at large. If there is one thing to learn from this problem, it is this: A system that perpetuates one-party dominance arouses the suspicions of many.     Stephen Harlin   Chadds Ford Township