See https://devel.trisquel.info/trisquel/package-helpers/blob/flidas/README.md for how to setup your system and https://trisquel.info/en/issues/8038 for some bugs you can work on.
And, of course, drop by at #trisquel-dev or send messages at https://listas.trisquel.info/mailman/listinfo/trisquel-devel
As for the documentation, any manual is valuable: https://listas.trisquel.info/mailman/listinfo/trisquel-devel
Just click an "Edit"!
