docs: explained setup:test (#391)

## PR Checklist

- [x] Addresses an existing open issue: fixes #278
- [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

Adds a testing docs section for `setup:test`, which is then also removed
as part of setup.

Is there anything else you'd suggest I document @emday4prez?

Author: JoshuaKGoldberg

.github/DEVELOPMENT.md

@@ -66,3 +66,19 @@ Calls to `console.log`, `console.warn`, and other console methods will cause a t
 
 This repository includes a [VS Code launch configuration](https://code.visualstudio.com/docs/editor/debugging) for debugging unit tests.
 To launch it, open a test file, then run _Debug Current Test File_ from the VS Code Debug panel (or press F5).
+
+### Testing the Setup Script
+
+In addition to unit tests, this template also includes an "end-to-end" test for `script/setup.js`.
+You can run it locally on the command-line:
+
+```shell
+pnpm run setup:test
+```
+
+That end-to-end test executes `script/setup-test-e2e.js`, which:
+
+1. Runs the setup script using `--skip-api`
+2. Checks that the local repository's files were changed correctly (e.g. removed setup-only files)
+
+As with the setup script itself, end-to-end tests are removed on package setup.

script/setup.js

@@ -373,6 +373,7 @@ try {
 					"./package.json",
 				],
 				[/## Explainer.*## Usage/gs, `## Usage`, "./README.md"],
+				[/\n### Testing the Setup Script.*$/gs, "", "./.github/DEVELOPMENT.md"],
 				[
 					`["src/index.ts!", "script/setup*.js"]`,
 					`"src/index.ts!"`,