Document what happens when the buffer isn't an image

[LinusU]

It would be good to add to the documentation that null is returned when an image format can't be found.

[sindresorhus]

That's already documented:

Returns an object (or null when no match) with:

https://github.com/sindresorhus/image-type#imagetypebuffer

[LinusU]

Oh, sorry, my bad