Add SPM Package.swift and SPM example project

Author: flowtoolz

Example Projects/SPMExample/.gitignore

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+.DS_Store
+/.build
+/Packages
+/*.xcodeproj

Example Projects/SPMExample/Package.resolved

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+// swift-tools-version:4.2
+
+import PackageDescription
+
+let package = Package(
+    name: "SPMExample",
+    dependencies: [
+        .package(url: "https://github.com/flowtoolz/SwiftObserver.git",
+                 .upToNextMajor(from: "5.0.0"))
+//        .package(path: "../../../SwiftObserver")
+    ],
+    targets: [
+        .target(name: "SPMExample",
+                dependencies: ["SwiftObserver"])
+    ],
+    swiftLanguageVersions: [.v4_2]
+)

Example Projects/SPMExample/Package.swift

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+print("Hello, world!")

Example Projects/SPMExample/Sources/SPMExample/main.swift

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+// swift-tools-version:4.2
+
+import PackageDescription
+
+let package = Package(
+    name: "SwiftObserver",
+    products: [
+        .library(name: "SwiftObserver",
+                 targets: ["SwiftObserver"]),
+    ],
+    dependencies: [
+        .package(
+            url: "https://github.com/flowtoolz/SwiftyToolz.git",
+            .upToNextMajor(from: "1.4.3")
+        ),
+    ],
+    targets: [
+        .target(name: "SwiftObserver",
+                dependencies: ["SwiftyToolz"],
+                path: "Code"),
+    ]
+)