Update readme to reflect NIO 1 backwards compat

Author: MrLotU

README.md

@@ -4,6 +4,15 @@
 
 A prometheus client for Swift supporting counters, gauges, histograms, summaries and info.
 
+# Installation
+
+SwiftPormetheus is available through SPM. To include it in your project add the following dependency to your `Package.swift`:
+```swift
+        .package(url: "https://github.com/MrLotU/SwiftPrometheus.git", from: "1.0.0-alpha")
+```
+
+_NOTE: For NIO 1 use `from: "0.0.0-alpha"` instead._
+
 # Usage
 
 To see a working demo, see [PrometheusExample](./Sources/PrometheusExample/main.swift).