Add PromiseKit dependency

- Added PromiseKit dependency

Carthage/Checkouts/PromiseKit/Extensions/Foundation/README.markdown

@@ -0,0 +1,46 @@
+# PromiseKit Foundation Extensions ![Build Status]
+
+This project adds promises to the Swift Foundation framework.
+
+We support iOS, tvOS, watchOS, macOS and Linux, Swift 3.0, 3.1, 3.2, 4.0 and 4.1.
+
+## CococaPods
+
+```ruby
+pod "PromiseKit/Foundation", "~> 6.0"
+```
+
+The extensions are built into `PromiseKit.framework` thus nothing else is needed.
+
+## Carthage
+
+```ruby
+github "PromiseKit/Foundation" ~> 3.0
+```
+
+The extensions are built into their own framework:
+
+```swift
+// swift
+import PromiseKit
+import PMKFoundation
+```
+
+```objc
+// objc
+@import PromiseKit;
+@import PMKFoundation;
+```
+
+## SwiftPM
+
+```swift
+let package = Package(
+    dependencies: [
+        .Package(url: "https://github.com/PromiseKit/Foundation.git", majorVersion: 3)
+    ]
+)
+```
+
+
+[Build Status]: https://travis-ci.org/PromiseKit/Foundation.svg?branch=master