Add a short how-to guide on access the headers corresponding to a specific pixel or world location

[Cadair]

If you want to know some metadata at a given pixel location you can do it like this:

ds[0,0,0].headers

and if you want to do it with a world coordinate you should be able to use crop to achieve the same thing.

We should have a very short how to guide on this.

requested by @tschad