docs(firestore-counter): use POSTINSTALL.md for client sample

Author: russellwheatley

firestore-counter/POSTINSTALL.md

@@ -20,7 +20,10 @@ match /databases/{database}/documents/pages/{page} {
 ```
 
 
-#### Specify a document path and increment value in your web app
+#### Client samples for incrementing counter and retrieving its value
+
+##### Web Client
+
 
 1.  Download and copy the [compiled client sample](https://github.com/firebase/extensions/blob/master/firestore-counter/clients/web/dist/sharded-counter.js) into your application project.
 
@@ -60,6 +63,55 @@ match /databases/{database}/documents/pages/{page} {
   </html>
   ```
 
+##### iOS Client
+
+1. Ensure your Swift app already has Firebase [initialized](https://firebase.google.com/docs/ios/setup).
+2. Copy and paste the sample [code](https://github.com/firebase/extensions/blob/next/firestore-counter/clients/ios/Sources/FirestoreCounter/FirestoreCounter.swift) and create this file  `FirestoreShardedCounter.swift` in the relevant directory you wish to use the `FirestoreShardedCounter` instance.
+
+```swift
+import UIKit
+import FirestoreCounter
+import FirebaseFirestore
+
+class ViewController: UIViewController {
+  // somewhere in your app code initialize Firestore instance
+  var db = Firestore.firestore()
+  // create reference to the collection and the document you wish to use 
+  var doc = db.collection("pages").document("hello-world")
+  // initialize FirestoreShardedCounter with the document and the property which will hold the counter value
+  var controller = FirestoreShardCounter(docRef: doc, field: "visits")
+
+  override func viewDidLoad() {
+    super.viewDidLoad()
+    // event listener which returns total amount
+    controller.onSnapshot { (value, error) in
+      if let error = error {
+        // handle error
+      } else if let value = value {
+        // 'value' param is total amount of pages visits
+      }
+    }
+  }
+
+  @IBAction func getLatest(_ sender: Any) {
+    // get current total
+    controller.get() { (value, error) in
+      if let error = error {
+        // handle error
+      } else if let value = value {
+        // 'value' param is total amount of pages visits
+      }
+    }
+  }
+
+  @IBAction func incrementer(_ sender: Any) {
+    // to increment counter
+    controller.incrementBy(val: Double(1))
+  }
+}
+
+```
+
 
 #### Upgrading from v0.1.3 and earlier