docs: add CJS/ESM emit entry (#986)

## PR Checklist

- [x] Addresses an existing open issue: fixes #942
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/create-typescript-app/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/create-typescript-app/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

I'm a little pleasantly surprised to remember that it's only two
steps...!

cspell.json

@@ -11,6 +11,7 @@
  "words": [
  "allcontributors",
  "apexskier",
+ "arethetypeswrong",
  "Codecov",
  "codespace",
  "commitlint",
@@ -32,7 +33,7 @@
  "tada",
  "tsup",
  "Unstaged",
- "wontfix",
- "vitest"
+ "vitest",
+ "wontfix"
  ]
 }

docs/FAQs.md

@@ -7,6 +7,53 @@ After you set up a repository, you can substitute in any tools you'd like.
 
 If you think the tool would be broadly useful to most consumers of this template, feel free to [file a feature request](https://github.com/JoshuaKGoldberg/create-typescript-app/issues/new?assignees=&labels=type%3A+feature&projects=&template=03-feature.yml&title=%F0%9F%9A%80+Feature%3A+%3Cshort+description+of+the+feature%3E) to add it in.
 
+## How can I add dual CommonJS / ECMAScript Modules emit?
+
+First, I'd suggest reading [TypeScript Handbook > Modules - Introduction](https://www.typescriptlang.org/docs/handbook/modules/introduction.html) to understand how CommonJS (CJS) and ECMAScript (ESM) came to be.
+
+Then:
+
+1. In `tsup.config.ts`, change the [tsup `format` option](https://tsup.egoist.dev/#bundle-formats) from `["esm"]` to `["cjs", "esm"]`
+2. Add a [`package.json` `"exports"` entry](https://nodejs.org/api/packages.html#subpath-exports) like:
+
+ <!-- eslint-disable jsonc/sort-keys -->
+
+ ```jsonc package.json
+ {
+ "exports": {
+ ".": {
+ "types": {
+ "import": "./lib/index.d.ts",
+ "require": "./lib/index.d.cts"
+ },
+ "import": "./lib/index.js",
+ "require": "./lib/index.cjs"
+ }
+ }
+ }
+ ```
+
+ <!-- eslint-enable jsonc/sort-keys -->
+
+That should be it!
+
+To be safe, consider checking with [arethetypeswrong](https://arethetypeswrong.github.io):
+
+1. Run `pnpm build`
+2. Run `npm pack`
+3. Upload that generated `.tgz` file to [arethetypeswrong.github.io](https://arethetypeswrong.github.io)
+
+### Why doesn't `create-typescript-app` have an option to dual emit CJS and ESM?
+
+Dual CJS/ESM emit is a stopgap solution while the JavaScript ecosystem migrates towards full ESM support in most-to-all popular user packages.
+Most packages newly created with `create-typescript-app` should target just ESM by default.
+
+Some packages published with `create-typescript` legitimately need dual CJS/ESM output because they're used by frameworks that don't yet fully support ESM.
+That's reasonable.
+
+Unless you know a package needs to support a CJS consumer, please strongly consider keeping it ESM-only (the `create-typescript-app` default).
+ESM-only packages have a smaller footprint by virtue of including fewer files.
+
 ## Is there a way to pull in template updates to previously created repositories?
 
 Not directly.