Support Apple's Swift Package Manager

**Reason**
- Main way to develop under Apple's platforms is using SPM

**Changes**
- Added Package.swift
- Added _SwiftPackageManagerFile.cpp to single_include/nlohmann in order SPM to see it as a C++ source file
- Added .build to .gitignore

**Context**
- SPM is a package manager for Swift and C-based languages. It is the main way to develop under Apple's platforms. It is also used to build Swift packages for Linux.

.gitignore

@@ -39,3 +39,6 @@
 /localhost.pem
 /localhost-key.pem
 /serve_header.yml
+
+# Swift Package Manager build directory
+/.build

Package.swift

@@ -0,0 +1,22 @@
+// swift-tools-version: 5.7
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+    name: "nlohmann-json",
+     platforms: [
+        .iOS(.v11), .macOS(.v10_13), .tvOS(.v11), .watchOS(.v4)
+    ],
+    products: [
+        .library(name: "nlohmann-json", targets: ["nlohmann-json"])
+    ],
+    targets: [
+        .target(
+            name: "nlohmann-json",
+            path: "single_include/nlohmann",
+            publicHeadersPath: "."
+        )
+    ],
+    cxxLanguageStandard: .cxx11
+)

README.md

@@ -1337,6 +1337,8 @@ If you are using [cget](https://cget.readthedocs.io/en/latest/), you can install
 
 If you are using [CocoaPods](https://cocoapods.org), you can use the library by adding pod `"nlohmann_json", '~>3.1.2'` to your podfile (see [an example](https://bitbucket.org/benman/nlohmann_json-cocoapod/src/master/)). Please file issues [here](https://bitbucket.org/benman/nlohmann_json-cocoapod/issues?status=new&status=open).
 
+If you are using [Swift Package Manager](https://swift.org/package-manager/), you can use the library by adding a package dependency to this repository. And target dependency as `.product(name: "nlohmann-json", package: "json")`.
+
 If you are using [NuGet](https://www.nuget.org), you can use the package [nlohmann.json](https://www.nuget.org/packages/nlohmann.json/). Please check [this extensive description](https://github.com/nlohmann/json/issues/1132#issuecomment-452250255) on how to use the package. Please file issues [here](https://github.com/hnkb/nlohmann-json-nuget/issues).
 
 If you are using [conda](https://conda.io/), you can use the package [nlohmann_json](https://github.com/conda-forge/nlohmann_json-feedstock) from [conda-forge](https://conda-forge.org) executing `conda install -c conda-forge nlohmann_json`. Please file issues [here](https://github.com/conda-forge/nlohmann_json-feedstock/issues).

single_include/nlohmann/apple/_SwiftPackageManagerFile.cpp

@@ -0,0 +1,3 @@
+// Umbrella source file for the Swift Package Manager
+
+ #include <json.hpp>