Add FirebaseFirestoreSwift readme (#5698)

* Add FirebaseFirestoreSwift readme

* fix nit

* address more review feedback

Author: morganchen12

Firestore/Swift/README.md

@@ -0,0 +1,38 @@
+# Firestore Swift Extensions
+
+This directory contains source files for Firestore's Swift extensions SDK.
+
+## Installation
+
+The FirebaseFirestoreSwift module is currently only available for installation
+through CocoaPods. To add FirebaseFirestoreSwift to your project, add the
+following to your Podfile
+
+```ruby
+pod 'FirebaseFirestoreSwift'
+```
+
+You will need to import the FirebaseFirestoreSwift module in any source file
+that depends on it, since it's not automatically bundled with the Firestore
+module.
+
+```swift
+import Firebase
+import FirebaseFirestoreSwift
+```
+
+The FirebaseFirestoreSwift module does not provide any additional utility to
+Objective-C projects, and therefore is not recommended for non-Swift projects.
+
+## Examples
+
+See the
+[Firestore quickstart sample](https://github.com/firebase/quickstart-ios/tree/master/firestore/FirestoreExample)
+and the
+[official Firebase documentation](https://firebase.google.com/docs/firestore/manage-data/add-data#custom_objects)
+for usage examples.
+
+## License
+
+The Firestore Swift Extensions SDK is available under the Apache-2 license. See
+the top-level LICENSE file for more details.