Add docs about GetBorderlessBox

[automata]

Since GetBorderlessBox is a heuristic based implementation, with many params and relations between image features (e.g. dimensions, intensities), would be interesting to add docs explaining both heuristics and examples of positive/negative cases (crop/don't crop).

[jonnor]

If you use fbp-spec you'll have a structured test format, which you can generate documentation from. ;)

[automata]

@jonnor great idea! Sounds like a good start for fbp-specing all image related tests!

@GabiThume ☝️

[jonnor]

This is quite related to the idea of a visual "datasheet" for a component, ref noflo/noflo-ui#51

[automata]

Oh wow that's cool. Really related, I can imagine some sketches/diagrams/images explaining the heuristics.