Update README

Signed-off-by: Andrej Mihajlov <and@mullvad.net>

README.md

@@ -32,6 +32,54 @@ $ open ./WireGuard/WireGuard.xcodeproj
 
 - Flip switches, press buttons, and make whirling noises until Xcode builds it.
 
+## WireGuardKit integration
+
+1. Open your Xcode project and add the Swift package with the following URL:
+   
+   ```
+   https://git.zx2c4.com/wireguard-apple
+   ```
+   
+   Choose `am/swiftpm` branch in the "Rules" section.
+   
+2. `WireGuardKit` links against `wireguard-go-bridge` library, but it cannot build it automatically
+   due to Swift package manager limitations. So it needs a little help from a developer. 
+   Please follow the instructions below to create a build target(s) for `wireguard-go-bridge`.
+   
+   - In Xcode, click File -> New -> Target. Switch to "Other" tab and choose "External Build 
+     System".
+   - Type in `WireGuardGoBridge<PLATFORM>` under the "Product name", replacing the `<PLATFORM>` 
+     placeholder with the name of the platform. For example, when targeting macOS use `macOS`, or 
+     when targeting iOS use `iOS`.
+     Make sure the build tool is set to: `/usr/bin/make` (default).
+   - In the appeared "Info" tab of a newly created target, type in the "Directory" path under 
+     the "External Build Tool Configuration":
+     
+     ```
+     $BUILD_DIR/../../SourcePackages/checkouts/wireguard-apple/wireguard-go-bridge
+     ```
+     
+   - Switch to "Build Settings" and find `SDKROOT`.
+     Type in `macosx` if you target macOS, or type in `iphoneos` if you target iOS.
+   
+3. Go to Xcode project settings and locate your network extension target and switch to 
+   "Build Phases" tab.
+   
+   - Locate "Dependencies" section and hit "+" to add `WireGuardGoBridge<PLATFORM>` replacing 
+     the `<PLATFORM>` placeholder with the name of platform matching the network extension 
+     deployment target (i.e macOS or iOS).
+     
+   - Locate the "Link with binary libraries" section and hit "+" to add `WireGuardKit`.
+   
+4. In Xcode project settings, locate your main bundle app and switch to "Build Phases" tab. 
+   Locate the "Link with binary libraries" section and hit "+" to add `WireGuardKit`.
+   
+5. iOS only: Locate Bitcode settings under your application target, Build settings -> Enable Bitcode, 
+   change the corresponding value to "No".
+   
+Note that if you ship your app for both iOS and macOS, make sure to repeat the steps 2-4 twice, 
+once per platform.
+
 ## MIT License
 
 Permission is hereby granted, free of charge, to any person obtaining a copy of