Why do you want to cite the depression?
Honestly If I were you and wanted to keep my employability as high as possible, i would be writing something like
"Dear xxxxxx,
I am writing to you to formally tender my resignation. My final working day will be (whatever you think it should be)
(if you have anything positive to say add it here)
Yours Sincerely (or faithfully if you have started the letter dear sir or dear madam)
Yourname here.
If you cite depression as a part of the reason for your resignation, while any firm who you apply to cannot use an illness issue as a reason not to employ you, it can be hard to prove and might go against you. Of course you will answer honestly any question put to you by a prospective employer but no point doing anything that might lessen your chances is there?