Add installation instructions

Author: nighthawk

README.md

@@ -8,9 +8,21 @@ It ~~has~~ will have various extension packages:
 
 - [GeoJSONKit+GEOS](https://gitlab.com/maparoni/geojsonkit-geos) for interfacing with the excellent [Geometry Engine - Open Source](https://trac.osgeo.org/geos)
 - GeoJSONKit+MapKit for usage in iOS / Mac apps
-- [GeoJSONKit+Turf](https://gitlab.com/maparoni/geojsonkit-turf) for spatial analysis, all in Swift
+- [GeoJSONKit+Turf](https://github.com/maparoni/geojsonkit-turf) for spatial analysis, all in Swift
 - [GeoJSONKit+Vapor](https://gitlab.com/maparoni/geojsonkit/snippets/1972906) for server-side usage
 
 Alternative packages:
 
 - [GEOSwift](https://github.com/GEOSwift/GEOSwift), which is *excellent* and has `Codable` support.
+
+## Installation
+
+### Swift Package Manager
+
+To install GeoJSONKit using the [Swift Package Manager](https://swift.org/package-manager/), add the following package to the `dependencies` in your Package.swift file:
+
+```swift
+.package(url: "https://github.com/maparoni/geojsonkit.git", from: "0.3.0"),
+```
+
+Then `import GeoJSONKit` in any Swift file in your module.