Add support for distribution via Swift Package Manager (SwiftPM) (#1073)

* Add Package.swift for distribution via SwiftPM

* Update publicHeadersPath

* Remove publicHeadersPath

* Flattened file structure

* Update project references

* Clean up

* Update README with instructions for installing via SwiftPM

* Fix typo

Co-authored-by: Mathias Claassen <[email protected]>

Co-authored-by: Mathias Claassen <[email protected]>

Author: eneko

Package.swift

@@ -0,0 +1,21 @@
+// swift-tools-version:5.2
+import PackageDescription
+
+let package = Package(
+    name: "XLForm",
+    platforms: [
+        .iOS(.v9)
+    ],
+    products: [
+        .library(name: "XLForm", targets: ["XLForm"])
+    ],
+    dependencies: [],
+    targets: [
+        .target(
+            name: "XLForm",
+            dependencies: [],
+            path: "XLForm",
+            publicHeadersPath: "XL"
+        )
+    ]
+)