Based on past experience, resignation letters should be 3 and only 3 sentences.
- I will be resigning my position at $company.
- My last date of work will be $date.
- My address is $address.
3.14 Sign and date the letter.
Reason for sentence 1: it is a resignation letter, there needs to be no possible misunderstanding about what is going on. It needs to be in writing as I've worked for bosses who deny that the person quitting was quitting so as to screw with them starting elsewhere.
Reason for sentence 2: I've worked for places that have "backdated" resignation letters and claimed you quit today - not in 2 weeks. Consequently, you may be expecting a paycheck that you'll never recieve. This is also why you date it with $today when you sign it.
Reason for sentence 3: Many people move, and if you need paperwork sent to you (such as retirement, health benefits, or your P45), there have been places I've worked at where they would deliberately and maliciously send your paperwork to an old address so that you cannot reply in the mandated 30 day period (because either it was still tied up in the post office's change of address system, or if that expired, returned to sender).
Never ever add stuff about why you are leaving. It is none of their business. If they ask, answer verbally, but never in any sort of writing.