docs: additional API docs (#100)

README.md

@@ -5,9 +5,13 @@
 [![CircleCI](https://circleci.com/gh/electron/universal/tree/main.svg?style=shield)](https://circleci.com/gh/electron/universal)
 [![NPM package](https://img.shields.io/npm/v/@electron/universal)](https://npm.im/@electron/universal)
 
-
 ## Usage
 
+This package takes an x64 app and an arm64 app and glues them together into a
+[Universal macOS binary](https://developer.apple.com/documentation/apple-silicon/building-a-universal-macos-binary).
+
+Note that parameters need to be **absolute** paths.
+
 ```typescript
 import { makeUniversalApp } from '@electron/universal';
 
@@ -18,20 +22,104 @@ await makeUniversalApp({
 });
 ```
 
+## Advanced configuration
+
+The basic usage patterns will work for most apps out of the box. Additional configuration
+options are available for advanced usecases.
+
+### Merging ASAR archives to reduce app size
+
+**Added in [v1.2.0](https://github.com/electron/universal/commit/38ab1c3559e25382957d608e49e624dc72a4409c)**
+
+If you are using ASAR archives to store your Electron app's JavaScript code, you can use the
+`mergeASARs` option to merge your x64 and arm64 ASAR files to reduce the bundle size of
+the output Universal app.
+
+If some files are present in only the x64 app but not the arm64 version (or vice-versa),
+you can exclude them from the merging process by specifying a `minimatch` pattern
+in `singleArchFiles`.
+
+```typescript
+import { makeUniversalApp } from '@electron/universal';
+
+await makeUniversalApp({
+  x64AppPath: 'path/to/App_x64.app',
+  arm64AppPath: 'path/to/App_arm64.app',
+  outAppPath: 'path/to/App_universal.app',
+  mergeASARs: true,
+  singleArchFiles: 'node_modules/some-native-module/lib/binding/Release/**', // if you have files in your asar that are unique to x64 or arm64 apps
+});
+```
+
+If `@electron/universal` detects an architecture-unique file that isn't covered by the
+`singleArchFiles` rule, an error will be thrown.
+
+### Skip lipo for certain binaries in your Universal app
+
+**Added in [1.3.0](https://github.com/electron/universal/commit/01dfb8a9636965fe154192b07934670dd42509f3)**
+
+If your Electron app contains binary resources that are already merged with the
+`lipo` tool, providing a [`minimatch`] pattern to matching files in the `x64ArchFiles`
+parameter will prevent `@electron/universal` from attempting to merge them a second time.
+
+```typescript
+import { makeUniversalApp } from '@electron/universal';
+
+await makeUniversalApp({
+  x64AppPath: 'path/to/App_x64.app',
+  arm64AppPath: 'path/to/App_arm64.app',
+  outAppPath: 'path/to/App_universal.app',
+  mergeASARs: true,
+  x64ArchFiles: '*/electron-helper', // `electron-helper` is a binary merged using `lipo`
+});
+```
+
+If `@electron/universal` detects a lipo'd file that isn't covered by the `x64ArchFiles` rule,
+an error will be thrown.
+
+### Including already codesigned app bundles into your Universal app
+
+**Added in [v1.4.0](https://github.com/electron/universal/commit/b02ce7697fd2a3c2c79e1f6ab6bf7052125865cc)**
+
+By default, the merging process will generate an `ElectronAsarIntegrity` key for
+any `Info.plist` files in your Electron app.
+
+If your Electron app bundles another `.app` that is already signed, you need to use
+the `infoPlistsToIgnore` option to avoid modifying that app's plist.
+
+```typescript
+import { makeUniversalApp } from '@electron/universal';
+
+await makeUniversalApp({
+  x64AppPath: 'path/to/App_x64.app',
+  arm64AppPath: 'path/to/App_arm64.app',
+  outAppPath: 'path/to/App_universal.app',
+  infoPlistsToIgnore: 'my-internal.app/Contents/Info.plist'
+});
+```
+
 ## FAQ
 
 #### The app is twice as big now, why?
 
-Well, a Universal app isn't anything magical.  It is literally the x64 app and
-the arm64 app glued together into a single application.  It's twice as big
-because it contains two apps in one.
+A Universal app is just the x64 app and the arm64 app glued together into a single application.
+It's twice as big because it contains two apps in one.
+
+Merging your ASAR bundles can yield significant app size reductions depending on how large
+your `app.asar` file is.
 
 #### What about native modules?
 
-The way `@electron/universal` works today means you don't need to worry about
-things like building universal versions of your native modules.  As long as
-your x64 and arm64 apps work in isolation the Universal app will work as well.
+Out of the box, you don't need to worry about building universal versions of your
+native modules. As long as your x64 and arm64 apps work in isolation, the Universal
+app will work as well.
+
+Note that if you are using `mergeASARs`, you may need to add architecture-specific
+binary resources to the `singleArchFiles` pattern.
+See [Merging ASARs usage](#merging-asar-archives-to-reduce-app-size) for an example.
 
 #### How do I build my app for Apple silicon in the first place?
 
-Check out the [Electron Apple silicon blog post](https://www.electronjs.org/blog/apple-silicon)
+Check out the [Electron Apple silicon blog post](https://www.electronjs.org/blog/apple-silicon).
+
+[`minimatch`]: https://github.com/isaacs/minimatch?tab=readme-ov-file#features