Update Migration Guide with ID getters and observer

Author: nan-li

MIGRATION_GUIDE.md

@@ -259,8 +259,12 @@ The user name space is accessible via `OneSignal.User` (in Kotlin) or `OneSignal
 | `val pushSubscription: IPushSubscription`                                     | `IPushSubscription getPushSubscription()`                                     | *The push subscription associated to the current user.*                                                                                                                                                                                  |
 | `fun setLanguage(value: String)`                                              | `void setLanguage(String value)`                                              | *Set the 2-character language either as a detected language or explicitly set for this user.*                                                                                                                                                |
 | `fun pushSubscription.addChangeHandler(handler: ISubscriptionChangedHandler)` | `void pushSubscription.addChangeHandler(ISubscriptionChangedHandler handler)` | *Adds a change handler that will run whenever the push subscription has been changed.*                                                                                                                                                   |
+| `val onesignalId: String`                                                     | `String getOnesignalId()`                                                     | *Returns the OneSignal ID for the current user, which can be the empty string if it is not yet available.*                                                                                                                               |
+| `val externalId: String`                                                      | `String getExternalId()`                                                      | *Returns the external ID for the current user, which can be the empty string if not set.*                                                                                                                                                |
+| `fun addObserver(observer: IUserStateObserver)`                               | `void addObserve(IUserStateObserver observer)`                                | *The `IUserStateObserver.onUserStateChange` method will be fired on the passed-in object when the user state changes. The User State contains the onesignalId and externalId (which can be empty strings), and the observer will be fired when these values change.*  |
+| `fun removeObserver(observer: IUserStateObserver)`                            | `void removeObserver(IUserStateObserver observer)`                            | *Remove a user state observer that has been previously added.*                                                                                                                                                                           |
 | `fun addAlias(label: String, id: String)`                                     | `void addAlias(String label, String id)`                                      | *Set an alias for the current user.  If this alias already exists it will be overwritten.*                                                                                                                                               |
-| `fun addAliases(aliases: Map<String, String>)`                                | `void addAliases(Map<String, String> aliases)`                                | S*et aliases for the current user. If any alias already exists it will be overwritten.*                                                                                                                                                  |
+| `fun addAliases(aliases: Map<String, String>)`                                | `void addAliases(Map<String, String> aliases)`                                | *Set aliases for the current user. If any alias already exists it will be overwritten.*                                                                                                                                                  |
 | `fun removeAlias(label: String)`                                              | `void removeAlias(String label)`                                              | *Remove an alias from the current user.*                                                                                                                                                                                                 |
 | `fun removeAliases(labels: Collection<String>)`                               | `void removeAliases(Collection<String> labels)`                               | *Remove multiple aliases from the current user.*                                                                                                                                                                                         |
 | `fun addEmail(email: String)`                                                 | `void addEmail(String email)`                                                 | *Add a new email subscription to the current user.*                                                                                                                                                                                      |