docs: mention granular PRs in CONTRIBUTING.md (#354)

## PR Checklist

- [x] Addresses an existing open issue: fixes #350
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/template-typescript-node-package/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/template-typescript-node-package/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

Slightly reorganizes the headings to make the sub-sections more
distinct.

Author: JoshuaKGoldberg

.github/CONTRIBUTING.md

@@ -41,6 +41,12 @@ Check [previously merged PRs](https://github.com/JoshuaKGoldberg/template-typesc
 If you don't think your PR is ready for review, [set it as a draft](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/changing-the-stage-of-a-pull-request#converting-a-pull-request-to-a-draft).
 Draft PRs won't be reviewed.
 
+#### Granular PRs
+
+Please keeep pull requests single-purpose: in other words, don't attempt to solve multiple unrelated problems in one pull request.
+Send one PR per area of concern.
+Multi-purpose pull requests are harder and slower to review, block all changes from being merged until the whole pull request is reviewed, and are difficult to name well with semantic PR titles.
+
 #### Pull Request Reviews
 
 When a PR is not in draft, it's considered ready for review.
@@ -54,5 +60,7 @@ Please try not to force-push commits to PRs that have already been reviewed.
 Doing so makes it harder to review the changes.
 We squash merge all commits so there's no need to try to preserve Git history within a PR branch.
 
+#### Post-Merge Recognition
+
 Once your PR is merged, if you haven't yet been added to the [_Contributors_ table in the README.md](../README.md#contributors) for its [type of contribution](https://allcontributors.org/docs/en/emoji-key "Allcontributors emoji key"), you should be soon.
 Please do ping the maintainer who merged your PR if that doesn't happen within 24 hours - it was likely an oversight on our end!