Add migration guide

MIGRATION.md

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+# Migration Guide
+
+## 17.x to 18.x
+
+### .NET Version
+
+This version of the plugin now requires .NET 7.0 (`net7.0-android` and `net7.0-ios`) as the min target framework.
+
+### Minimum iOS Version
+
+This version of the plugin now requires iOS 14+ as the min deployment target and Xcode 14.3+.
+
+### iOS Log Levels
+
+The `TRACE` level has been renamed to `VERBOSE`, for consistency with other platforms/frameworks.
+
+### API Changes
+
+#### Methods
+
+| 17.x | 18.x |
+|------|------|
+| `Airship.Instance.NamedUser = "some named user ID";` | `Airship.Instance.IdentifyContact("some named user ID");` |
+| `Airship.Instance.NamedUser = null;` | `Airship.Instance.ResetContact();` |
+| `var namedUser = Airship.Instance.NamedUser;` | `Airship.Instance.GetNamedUser(namedUser => { ... });` |
+| `Airship.Instance.EditNamedUserTagGroups();` | `Airship.Instance.EditContactTagGroups();` |
+| `Airship.Instance.EditNamedUserAttributes();` | `Airship.Instance.EditContactAttributes();` |
+| `var messages = Airship.Instance.InboxMessages;` | `Airship.Instance.InboxMessages(messages => { ... });` |
+| `var count = Airship.Instance.MessageCenterUnreadCount;` | `Airship.Instance.MessageCenterUnreadCount(count => { ... });` |
+| `var count = Airship.Instance.MessageCenterCount;` | `Airship.Instance.MessageCenterCount(count => { ... });` |
+
+### API Additions
+
+#### Push notification status Listener
+
+```csharp
+Airship.Instance.OnPushNotificationStatusUpdate -= OnPushNotificationStatusEvent;
+
+private void OnPushNotificationStatusEvent(object sender, PushNotificationStatusEventArgs e) => 
+{
+	bool isUserNotificationsEnabled = e.IsUserNotificationsEnabled;
+	// ...
+};
+```
+
+#### Editing Channel Subscription Lists
+
+```csharp
+Airship.Instance.EditChannelSubscriptionLists()
+    .subscribe("food");
+    .unsubscribe("sports");
+    .apply();
+```
+
+#### Editing Contact Subscription Lists
+
+```csharp
+Airship.Instance.EditContactSubscriptionLists()
+    .subscribe("food", "app")
+    .unsubscribe("sports", "sms")
+    .apply()
+```
+
+### API Removals
+
+#### `Airship.Instance.OnChannelUpdate`
+
+Replace with either `OnChannelCreation` or `OnPushNotificationStatusUpdate`, depending on usage.