docs: document app.plugin.js in package.json exports (#834)

As people start using package.json exports more, it might be worth
highlighting the "usual" exports people might want to have, which
includes expo config plugin files (app.plugin.js).

Feel free to close if you see this being out of scope.

Thank you :)

Co-authored-by: Satyajit Sahoo <satyajit.happy@gmail.com>

Author: vonovak

docs/pages/esm.md

@@ -64,8 +64,22 @@ The `./package.json` field is used to point to the library's `package.json` file
 
 Using the `exports` field has a few benefits, such as:
 
-- It [restricts access to the library's internals](https://nodejs.org/api/packages.html#main-entry-point-export) by default. You can explicitly specify which files are accessible with [subpath exports](https://nodejs.org/api/packages.html#subpath-exports).
 - It allows you to specify different entry points for different environments with [conditional exports](https://nodejs.org/api/packages.html#conditional-exports) (e.g. `node`, `browser`, `module`, `react-native`, `production`, `development` etc.).
+- It [restricts access to the library's internals](https://nodejs.org/api/packages.html#main-entry-point-export) by default. You can explicitly specify which files are accessible with [subpath exports](https://nodejs.org/api/packages.html#subpath-exports).
+
+  So make sure to explicitly specify any files that need to be readable by other tools, e.g. `./app.plugin.js` if you provide a [Expo Config plugin](https://docs.expo.dev/config-plugins/plugins-and-mods/#apppluginjs):
+
+  ```diff
+  "exports": {
+    ".": {
+      "source": "./src/index.tsx",
+      "types": "./lib/typescript/src/index.d.ts",
+      "default": "./lib/module/index.js"
+    },
+    "./package.json": "./package.json",
+  + "./app.plugin.js": "./app.plugin.js"
+  },
+  ```
 
 ### A note on `import.meta`