Add Package.swift (#39)

* Add Package.swift for LLM and MNIST
* Make ModelType properties public
* Make ModelType method createModel public
* Add installation instructions to readme

Package.swift

@@ -0,0 +1,58 @@
+// swift-tools-version: 5.9
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+    name: "mlx-libraries",
+    platforms: [.macOS(.v14), .iOS(.v16)],
+    products: [
+        .library(
+            name: "LLM",
+            targets: ["MLXLLM"]),
+        .library(
+            name: "MNIST",
+            targets: ["MLXMNIST"]),
+    ],
+    dependencies: [
+        .package(url: "https://github.com/ml-explore/mlx-swift", branch: "main"),
+        .package(url: "https://github.com/huggingface/swift-transformers", from: "0.1.5"),
+        .package(url: "https://github.com/1024jp/GzipSwift", from: "6.0.1"),
+        .package(url: "https://github.com/apple/swift-async-algorithms", from: "1.0.0"),
+    ],
+    targets: [
+        .target(
+            name: "MLXLLM",
+            dependencies: [
+                .product(name: "MLX", package: "mlx-swift"),
+                .product(name: "MLXFast", package: "mlx-swift"),
+                .product(name: "MLXNN", package: "mlx-swift"),
+                .product(name: "MLXRandom", package: "mlx-swift"),
+                .product(name: "Transformers", package: "swift-transformers"),
+                .product(name: "AsyncAlgorithms", package: "swift-async-algorithms"),
+            ],
+            path: "Libraries/LLM",
+            exclude: [
+                "README.md",
+                "LLM.h",
+            ]
+        ),
+        .target(
+            name: "MLXMNIST",
+            dependencies: [
+                .product(name: "MLX", package: "mlx-swift"),
+                .product(name: "MLXFast", package: "mlx-swift"),
+                .product(name: "MLXNN", package: "mlx-swift"),
+                .product(name: "MLXRandom", package: "mlx-swift"),
+                .product(name: "Transformers", package: "swift-transformers"),
+                .product(name: "AsyncAlgorithms", package: "swift-async-algorithms"),
+                .product(name: "Gzip", package: "GzipSwift"),
+            ],
+            path: "Libraries/MNIST",
+            exclude: [
+                "README.md",
+                "MNIST.h",
+            ]
+        ),
+    ]
+)