Docs: MyPandocOpen custom_open - use existing viewer instance

[ccaprani]

I use this mostly for writing presentations in markdown files for beamer. I've used the custom_open hook to open the rendered PDF, but it's a bit of a nuisance that it opens a new instance of the viewer each time.

I use zathura, and I came across this and adapted it, and it could be a useful addition to the docs, at least for zathura and/or markdown users:

function! MyPandocOpen(file)
        let position = line('.') . ":" . col('.') . ":" . expand('%:p') . " "
        return "zathura -x 'nvr --remote +%{line} %{input}' --synctex-forward " . position . " " . substitute(expand('%:p'),"md$","pdf", "")
        "" return 'xdg-open '. a:file
endfunction