Add unbox_message filter & info in readme on usage.

@chriswallace Take a look, this is what I was referring to earlier in today's theme meeting.. I like the idea of keeping the first paragraph the same but giving a bit of control for the second paragraph. 

While I think this will be sufficient for the time being, we might want to change this up to support list items that can be filtered too. 

Let me know what you think, merge if you want.