Add fileRemoved, elaborate on filesSubmitted.

Add fileRemoved.
Elaborate on filesSubmitted to note that since it happens after the file is added to the upload queue, invoking flow.upload() will upload the files in the list.

Author: SuberFu

README.md

@@ -188,7 +188,8 @@ this means that calling `flow.upload()` function will not start current file upl
 Optionally, you can use the browser `event` object from when the file was
 added.
 * `.filesAdded(array, event)` Same as fileAdded, but used for multiple file validation.
-* `.filesSubmitted(array, event)` Can be used to start upload of currently added files.
+* `.filesSubmitted(array, event)` Same as filesAdded, but happens after the file is added to upload queue. Can be used to start upload of currently added files.
+* `.fileRemoved(file)` The specific file was removed from the upload queue. Combined with filesSubmitted, can be used to notify UI to update its state to match the upload queue.
 * `.fileRetry(file, chunk)` Something went wrong during upload of a specific file, uploading is being 
 retried.
 * `.fileError(file, message, chunk)` An error occurred during upload of a specific file.