Update readme

README.md

@@ -1,7 +1,10 @@
 MailerLite Swift SDK
 ====================
+A basic Swift SDK for the [MailerLite API](https://developers.mailerlite.com/docs/).
 
 ## Usage
+
+### Get a list of subscribers
 ```swift
 import MailerLiteApiClient
 
@@ -18,6 +21,13 @@ mailerLite.send(ListSubscribers()) { result in
         // TODO: handle error
     }
 }
+```
+
+### Upsert a subscriber
+```swift
+import MailerLiteApiClient
+
+let mailerLite = MailerLiteAPIClient(apiKey: "your-api-key")
 
 mailerLite.send(UpsertSubscriber(email: "dummy@example.com", fields: Subscriber.Fields(lastName: "Testerson"))) { result in
     switch result {
@@ -32,5 +42,38 @@ mailerLite.send(UpsertSubscriber(email: "dummy@example.com", fields: Subscriber.
 }
 ```
 
+### Update a subscriber
+```swift
+import MailerLiteApiClient
+
+let mailerLite = MailerLiteAPIClient(apiKey: "your-api-key")
+
+mailerLite.send(UpdateSubscriber(
+    id: "31897397363737859",
+    fields: Subscriber.Fields(
+        lastName: nil,
+        name: "Dummy"
+    ),
+    groups: [
+        "4243829086487936",
+        "14133878422767533",
+        "31985378335392975"
+    ]
+))) { result in
+    switch result {
+    case .success(let response):
+        // TODO: handle response
+        let data = response.data {
+            print("Subscriber \(data.id) updated")
+        }
+    case .failure(let error):
+        // TODO: handle error
+    }
+}
+```
+
+## Contributing
+Please help improve this SDK! Contributions welcome.
+
 ## License
 [The MIT License (MIT)](LICENSE)