Docs claim to support vim-plug, but I could not get it to work by default. I fixed zvim#plug#add() and s:install_manager to at least work minimally. Still unfixed is that using vim-plug does not seem to respect lazy-loading rules.
close https://github.com/Shougo/dein.vim/issues/177
issue opened here: https://github.com/google/vimdoc/issues/94