Add doc about keeping a strong reference (#231)

* Add doc about keeping a strong reference

* Move the message

Author: TomKemp

README.md

@@ -335,6 +335,8 @@ Pusher *pusher = [[Pusher alloc] initWithAppKey:@"YOUR_APP_KEY"];
 
 This returns a client object which can then be used to subscribe to channels and then calling `connect()` triggers the connection process to start.
 
+**Important:** You must keep a strong reference to the `Pusher` client. You could achieve that by making `pusher` a property of your app delegate, for example.
+
 You can also set a `userDataFetcher` on the connection object.
 
 - `userDataFetcher (() -> PusherPresenceChannelMember)` - if you are subscribing to an authenticated channel and wish to provide a function to return user data