documentation: how to avoid wrong/different baselines for some characters

[abubelinha]

In my first tests, I noticed all characters in the same line are surrounded by boxes of similar heights (based on the top and bottom black pixels found in that line, according to the segmentation logic you comment in #11).
Other characters written in different lines will be surrounded by boxes of different heights (for example, if uppercase and lowercase letters were in different lines). That produced this kind of effect in my first generated font:

I guess this issue could be avoided using some trick like the one explained here:
https://github.com/osp/osp.tools.fons#baseline

Baseline

"When importing a bitmap, Fontforge scales up/down the bitmap so that it fits into the glyph box. If we use GlyphTracer 1.4, this ends up with different scales of glyphs. In order to prevent this, we can put a black rectangle of the height of the maximum ascendant and maximum descendant"

@jpakkane I don't know if you agree or there is an easier solution.
Whatever it is, I'd suggest adding it to documentation, so users don't realize of this after having done all the hard work.

BTW. Thanks a lot for this great software!
@abubelinha