Tl;dr is an antipattern, and it's easy to see why.

Lead with a strong point. Pique youre reader's interest. Then, and only then, will they lap up your wall of text. Tl;dr is a gag, a joke, mockery of bad writing style. If you find yourself using it in a serious way, as a crutch, stop, rewrite, don't go down that path.

TL;DR doesn't always appear at the beginning which is what cmdkeen was talking about.