Add object detection processing tutorial
_The documentation is not available anymore as the PR was closed or merged._ Great idea! Now that we have more than one task, it makes sense to separate image classification and object detection so it'll be easier for users to follow. @lhoestq do we want to do that in this PR, or should we merge it and let @stevhliu reorganize separately? 
The following adds a quick guide on how to process object detection datasets with `albumentations`. 