Add SPM support

Author: pomozoff

Package.resolved

@@ -0,0 +1,23 @@
+// swift-tools-version:5.1
+import PackageDescription
+
+let package = Package(
+    name: "NSObject-Rx",
+    platforms: [
+        .iOS(.v8),
+    ],
+    products: [
+        .library(name: "NSObject-Rx", targets: ["NSObject-Rx"]),
+    ],
+    dependencies: [
+        .package(url: "https://github.com/ReactiveX/RxSwift", from: "5.0.1"),
+    ],
+    targets: [
+        .target(
+            name: "NSObject-Rx",
+            dependencies: ["RxSwift"],
+            path: ".",
+            sources: ["NSObject+Rx.swift"]
+        ),
+    ]
+)