Add comments about what to contribute to mathlib

templates/contribute/index.md

@@ -10,10 +10,21 @@ to make the process of contributing as smooth as possible.
    - The explanation of [naming conventions](naming.html).
    - The [documentation guidelines](doc.html).
 
-Once you have code that you'd like to contribute, you should open a PR.
+## What to contribute to mathlib
+
+The first question you will need to consider is whether the material you want to contribute is a good fit for `mathlib`. Whilst there is no formal description of exactly what mathlib's remit is, here are some questions which you can ask about your proposed contribution.
+
+* Is the material typically covered in an undergraduate or graduate level course in a research mathematics department? If not, then the material may not in scope for `mathlib`.
+* Is the topic of the material contained within the [mathematical interests of the `mathlib` maintainers](https://github.com/leanprover-community/mathlib4?tab=readme-ov-file#maintainers)? If not, then the maintainers might find your code hard to maintain as lean and `mathlib` evolve over time, which again makes it not a good fit for `mathlib`.
+
+In particular the remit of mathlib should *not* be thought of as "all of mathematics and related areas". As the number of open PRs increases, the maintainers will sometimes need to make some hard decisions.
+
+If it is not completely clear to you that the code in your project is a good fit for mathlib, you might want to consider creating a standalone repository, and adding `mathlib` as a dependency. There are many Lean repositories on github, indexed by [reservoir](https://reservoir.lean-lang.org). This solution is a particularly good fit for projects in areas which do not align with the expertise of the mathlib maintainers.
 
 ## Working on mathlib
 
+If you believe that your code does belong in mathlib, you should open a PR.
+
 We use `git` to manage and version control `mathlib`.
 The `master` branch is the "production" version of mathlib.
 It is essential that everything in the master branch compiles without errors, and there are no `sorry`s.