Maybe you’re thinking about writing something and contributing to the stacks project. Here are some thoughts I have about this.
(1) Of course it would be fantastic to get your lecture notes, preprint, etc as a contribution!
(2) Please make sure you have copyright over the material and that you agree to release it under the GFDL
(3) Right now most of the material in the stacks project is really building theory and not so much giving an overview of how to get from A to B. We try almost everywhere to prove lemmas in the correct generality. I want to have (a) more chapters where the material gets discussed in a way that it becomes usable for those not interested in building foundations, (b) more expository material, and (c) more alternative approaches to theorems and foundations. Thus it is quite possible that your material would be welcome even if it has already been covered in the stacks project.
(4) Whenever anybody contributes a text to the stacks project I get to edit it and decide which pieces to delete or which things to add. This is because right now I am the maintainer of the project. Even if I use only 2 pages out of a 5 page manuscript I would still be ecstatic about it.
(5) Being able to copy and paste things literally from your manuscript (without having to retype them) can be a great help. I have no compulsion about taking things from the literature and writing them up, but I worry about using very recent manuscripts of people. (For example, I don’t scour the arXiv looking for tidbits to copy and paste.)
(6) Anybody can take a copy of the stacks project and run a competing “stacks project”. The license allows that! I doubt this will happen, but it could happen. I would kind of enjoy that, but you might not enjoy Mr X randomly editing a manuscript you worked hard on (of course this might happen if I edit it too).
The motivation for writing something should be that you want to explain something more clearly than in the literature, explain a particular technical point, explain something in a new way, prove a lemma you just realized is true but you’ve never seen before, prove a new theorem, or you simply want to wrok through something in order to understand it better. You should publish your write-ups on the web with your name on them (i.e., put it on your web-page, dump it on the arXiv and/or submit it to a journal). But once you’ve written something, you’ve published it, and you’re willing (and able) to share it, then just send it over and I’ll see if (parts of) it can be incorporated.