Add UpdateCheck and some supporting code

README.md

@@ -16,7 +16,55 @@ Here's how it works:
 
 - The self-updater will download the .zip file and validate it using the checksum provided in the manifest file. If the checksum is valid, the app is (re)placed in `/Applications` and finally launched.
 
-## Example
+## Checking for updates
+
+### Requirements
+
+To check for updates in your main target you need to meet a few conditions:
+
+1. You must have a self-updater app that can be executed separately. You must build and embed this app as part of the main target. It's relatively easy to do this, see the *Self-Updater* section below for instructions on how to make the self-updater work correctly.
+
+2. You must declare where the temporary directory is for the updater.
+
+3. You must have a CaskFile and zip hosted which will both be downloaded and checked if the user searches for updates.
+
+### Making a manifest file
+
+AppUpdater uses the same format Cask files for Homebrew use. A valid CaskFile looks like this, for example:
+
+```
+cask 'my-app' do
+  version '1.0_95'
+  sha256 '1b39bf7977120222e5bea2c35414f76d1bb39a882334488642e1d4e87a48c874'
+
+  url 'https://my-app.test/latest/app.zip'
+  name 'My App'
+  homepage 'https://myapp.test'
+
+  app 'My App.app'
+end
+```
+
+You must calculate the SHA-256 hash of the .zip file, since that will be validated. The app will also look at the version number to compare to the installed version. 
+
+The version number uses the following format: VERSION_BUILD. (So for this example we are looking at a manifest of My App, version 1.0, build 95.)
+
+You must always place the CaskFile at the same URL, and you will specify where to find this file via the `caskUrl` parameter as part of searching for updates.
+
+### How to perform an update check
+
+To check for updates, simply create a new `UpdateCheck` instance with the correct configuration, and call `perform()`:
+
+```swift
+await UpdateCheck(
+    selfUpdaterName: "MyApp Self-Updater.app",
+    selfUpdaterDirectory: "~/.config/com.example.my-app/updater",
+    caskUrl: URL(string: "https://my-app.test/latest/build.rb")!,
+    promptOnFailure: true
+).perform()
+```
+
+## Self-Updater
 
 As a separate target (for a macOS app), you need to add the following file:
 
@@ -33,3 +81,5 @@ let delegate = SelfUpdater(
 NSApplication.shared.delegate = delegate
 _ = NSApplicationMain(CommandLine.argc, CommandLine.unsafeArgv)
 ```
+
+You must then make sure that this app is included as a sub-app for the main target. It needs to be referenced correctly as part of the `selfUpdaterName` parameter of `UpdateCheck` (see the previous section).