[Feauture request] Add training doc

[bropines]

I would like to train MangaOCR on a set of manga but in English and other languages. Yes, this contradicts the very logic of mangaOCR, but in times when it is faster to find works on English sites, but need to be translated to some other site, this would be very convenient. If possible, I could provide a dataset from such a manga.

There is also a big problem with mangaOCR fonts. It can collect a couple of dozen fonts and make an automatic dataset from them for recognition, as well as make “distortions” so that the neural network learns to recognize even in conditions of very incomprehensible text