Document usage of @custom attribute

[kba]

The script has assumptions on the conventional usage of @custom in PAGE XML based on how it's used in Transkribus (?). It would be helpful to document this, as well as the supported @types.

[skurzinz]

I second this.

Recently, the Transkribus web app started offering a Wikidata id widget in their transcription interface, which is saved in the PAGE as in

<TextLine id="tr_3_tl_6" custom="readingOrder {index:14;} person {offset:6; length:13;wikiData:Q43388312;}">

@dariok would it make sense to output this, again as a @key or @ref into the resulting TEI?