Add PromiseKit dependency

- Added PromiseKit dependency

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

@@ -0,0 +1,34 @@
+# PromiseKit Photos Extensions ![Build Status]
+
+This project adds promises to Apple’s Photos framework.
+
+## CocoaPods
+
+```ruby
+pod "PromiseKit/Photos", "~> 6.0"
+```
+
+The extensions are built into `PromiseKit.framework` thus nothing else is needed.
+
+## Carthage
+
+```ruby
+github "PromiseKit/Photos" ~> 3.0
+```
+
+The extensions are built into their own framework:
+
+```swift
+// swift
+import PromiseKit
+import PMKPhotos
+```
+
+```objc
+// objc
+@import PromiseKit;
+@import PMKPhotos;
+```
+
+
+[Build Status]: https://travis-ci.org/PromiseKit/Photos.svg?branch=master