Add more README content

README.md

@@ -6,3 +6,27 @@ eliminates the need for `#if os` imports.
 This package pretends to be part of [apple/swift-system](), and aspire to be subsumed by it one day.
 
 [apple/swift-system]: https://github.com/apple/swift-system
+
+## Install
+
+This is a standard SwiftPM package. Include it the standard way in your manifest:
+
+```swift
+let package = Package(
+    // name, platforms, products, etc.
+    dependencies: [
+        .package(url: "https://github.com/dduan/swift-system-exit", from: "1.0.0"),
+        // other dependencies
+    ],
+    targets: [
+        .target(name: "MyTarget", dependencies: [
+            .product(name: "SystemExit", package: "swift-system-exit"),
+        ]),
+        // other targets
+    ]
+)
+```
+
+## LICENSE
+
+See [LICENSE](LICENSE.txt) for license information.