You may have done your letter by now, but when I quit my job in January I had to write a letter as well. Mine was short and sweet and went something like this:
This letter is to inform you of my intent to resign from my current position effective *date*. I have enjoyed my experience working here and, if in the future, I wish to resume my role in this company I would like to be considered for the position.
Keep it short.
If you want to ever be considered for another job there, mention that you enjoyed your job and the people you worked with.
It's not necessary to go beyond that. If they ask, you might give a reason for leaving...make sure it's reasonable and doesn't reflect badly on the company.
If you want to, you can certainly add it to the letter, but it isn't necessary.
What IS NECESSARY (esp. If you ever want to be considered for rehire...or want a reference from them) is as much notice as possible. 2 weeks is great.
If you are a professional, make it a month.