Add README.md with simple code example

README.md

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+# Systemd
+
+A simple Swift library to interface with systemd on Linux.
+
+## Installation
+
+Add the following dependency to your `Package.swift` file:
+
+```swift
+.package(url: "https://github.com/xtremekforever/swift-systemd.git", from: "0.0.1")
+```
+
+Then, add it to your target `dependencies` section like this:
+
+```swift
+.product(name: "Systemd", package: "swift-systemd")
+```
+
+## Usage
+
+For now, the library only has the ability to see if the app is running under systemd. To do this,
+`import Systemd` in your app, then use `SystemdHelpers` to get a true or false value:
+
+```swift
+if SystemdHelpers.isSystemdService {
+    print("This app is running as a systemd service!")
+
+    // do things like modify logging format (if using swift-log) or whatever else is needed.
+}
+```