Added notes to readme about handling errors

Author: hamchapman

README.md

@@ -43,6 +43,7 @@ chan.trigger("client-test", data: ["test": "some value"])
 * [Binding to events](#binding-to-events)
   * [Globally](#global-events)
   * [Per-channel](#per-channel-events)
+  * [Receiving errors](#receiving-errors)
 * [Presence channel specifics](#presence-channel-specifics)
 * [Testing](#testing)
 * [Communication](#communication)
@@ -258,6 +259,45 @@ myChannel.bind("new-price", callback: { (data: AnyObject?) -> Void in
 })
 ```
 
+### Receiving errors
+
+Errors are sent to the client for which they are relevant with an event name of `pusher:error`. These can be received and handled using code as follows. Obviously the specifics of how to handle them are left up to the developer but this displays the general pattern.
+
+```swift
+pusher.bind({ (message: AnyObject?) in
+    if let message = message as? [String: AnyObject], eventName = message["event"] as? String where eventName == "pusher:error" {
+        if let data = message["data"] as? [String: AnyObject], errorMessage = data["message"] as? String {
+            print("Error message: \(errorMessage)")
+        }
+    }
+})
+```
+
+The sort of errors you might get are:
+
+```bash
+# if attempting to subscribe to an already subscribed-to channel
+
+"{\"event\":\"pusher:error\",\"data\":{\"code\":null,\"message\":\"Existing subscription to channel presence-channel\"}}"
+
+# if the auth signature generated by your auth mechanism is invalid
+
+"{\"event\":\"pusher:error\",\"data\":{\"code\":null,\"message\":\"Invalid signature: Expected HMAC SHA256 hex digest of 200557.5043858:presence-channel:{\\\"user_id\\\":\\\"200557.5043858\\\"}, but got 8372e1649cf5a45a2de3cd97fe11d85de80b214243e3a9e9f5cee502fa03f880\"}}"
+```
+
+You can see that the general form they take is:
+
+```bash
+{
+  "event": "pusher:error",
+  "data": {
+    "code": null,
+    "message": "Error message here"
+  }
+}
+```
+
+
 ### Unbind event handlers
 
 You can remove previously-bound handlers from an object by using the `unbind` function. For example,