Basic of a Resignation Letter. There's no need to sugarcoat or get creative in the beginning; just state the position you're resigning from and the effective date. While you probably shared with your boss your reasons for leaving, you don't need to describe them here__keeping it simple is perfectly fine.
Sure, it may sit in the depths of HR until the end of time, but you can be sure of two things: One, your former boss (and future reference) will read it and be impressed. And two, if you ever decide to come back to your company (hey, it happens), it's a very good thing that last thing on file is a great, professionally written resignation letter.