Add README.

Author: scottrhoyt

README.md

@@ -0,0 +1,143 @@
+# Swift implementation of the geodesic routines in GeographicLib
+
+This is a Swift library to solve geodesic problems on an ellipsoid model of
+the earth.
+
+This is a Swift wrapper around the C implementation of the geodesic routines
+from [GeographicLib](https://geographiclib.sourceforge.io).
+
+Licensed under the MIT/X11 License; see
+[LICENSE.txt](https://geographiclib.sourceforge.io/LICENSE.txt).
+
+The algorithms are documented in
+
+* C. F. F. Karney,
+  [Algorithms for geodesics](https://doi.org/10.1007/s00190-012-0578-z),
+  J. Geodesy **87**(1), 43–55 (2013);
+  [Addenda](https://geographiclib.sourceforge.io/geod-addenda.html).
+
+## Other links:
+
+* Library documentation: (coming soon)
+* GeographicLib: https://geographiclib.sourceforge.io
+* C implementation: https://github.com/geographiclib/geographiclib-c
+
+## Installation
+
+### Swift Package Manager
+
+Add the following to your `Package.swift` file:
+
+```swift
+dependencies: [
+    .package(url: "https://github.com/scottrhoyt/geographiclib-swift.git", from: "1.0.0")
+]
+```
+
+## Usage
+
+### Basic Usage
+
+```swift
+import GeographicLib
+
+// Create a geodesic calculator with WGS-84 ellipsoid (default)
+let geodesic = Geodesic()
+
+// Solve the direct problem: given a starting point, azimuth, and distance
+let direct = geodesic.direct(
+    latitude: 40.64,      // JFK Airport
+    longitude: -73.78,
+    azimuth: 45.0,        // northeast
+    distance: 10_000_000  // 10,000 km
+)
+print("Destination: \(direct.latitude)°, \(direct.longitude)°")
+
+// Solve the inverse problem: given two points, find distance and azimuths
+let inverse = geodesic.inverse(
+    latitude1: 40.64,     // JFK Airport
+    longitude1: -73.78,
+    latitude2: 1.36,      // Singapore Changi Airport
+    longitude2: 103.99
+)
+print("Distance: \(inverse.distance) meters")
+print("Initial azimuth: \(inverse.azimuth1)°")
+```
+
+### Using Different Ellipsoids
+
+```swift
+// Use GRS-80 ellipsoid
+let grs80 = Geodesic(.grs80)
+
+// Use a custom ellipsoid
+let customEllipsoid = Ellipsoid(equatorialRadius: 6378000.0, flattening: 1.0/300.0)
+let customGeodesic = Geodesic(customEllipsoid)
+
+// Use a sphere for simplified calculations
+let sphere = Geodesic(.sphere)
+```
+
+### Geodesic Lines
+
+For efficient calculations of multiple points along a geodesic:
+
+```swift
+// Create a geodesic line
+let line = geodesic.inverseLine(
+    latitude1: 40.64, longitude1: -73.78,  // JFK
+    latitude2: 1.36, longitude2: 103.99     // Singapore
+)
+
+// Calculate waypoints
+for i in 0...10 {
+    let fraction = Double(i) / 10.0
+    let position = line.position(distance: line.distance * fraction)
+    print("Waypoint \(i): \(position.latitude)°, \(position.longitude)°")
+}
+```
+
+### Polygon Areas
+
+Calculate areas and perimeters of geodesic polygons:
+
+```swift
+// Simple polygon area calculation
+let antarctica = [
+    (-72.9, -74), (-71.9, -102), (-74.9, -102), (-74.3, -131),
+    (-77.5, -163), (-77.4, 163), (-71.7, 172), (-65.9, 140),
+    (-65.7, 113), (-66.6, 88), (-66.9, 59), (-69.8, 25),
+    (-70.0, -4), (-71.0, -14), (-77.3, -33), (-77.9, -46), (-74.7, -61)
+]
+
+let (area, perimeter) = geodesic.polygonArea(
+    latitudes: antarctica.map { $0.0 },
+    longitudes: antarctica.map { $0.1 }
+)
+print("Antarctica area: \(area) m²")
+print("Antarctica perimeter: \(perimeter) m")
+
+// Using the Polygon type for more control
+var polygon = Polygon()
+polygon.addPoint(latitude: 0, longitude: 0)
+polygon.addPoint(latitude: 0, longitude: 90)
+polygon.addPoint(latitude: 90, longitude: 0)
+
+let result = polygon.compute()
+print("Triangle area: \(result.area!) m²")
+```
+
+## Features
+
+- **Direct geodesic problem**: Given a starting point, azimuth, and distance, find the destination point
+- **Inverse geodesic problem**: Given two points, find the distance and azimuths between them
+- **Geodesic lines**: Efficiently compute multiple points along a geodesic
+- **Polygon areas**: Calculate areas and perimeters of geodesic polygons
+- **Multiple ellipsoids**: Support for WGS-84, GRS-80, custom ellipsoids, and spheres
+- **High precision**: Accurate to round-off for distances up to 180°
+- **Thread-safe**: All types are immutable and `Sendable`
+
+## Requirements
+
+- Swift 5.5 or later
+- Platforms: macOS, iOS, tvOS, watchOS, Linux, Windows