Inspired by Managing Your Dotfiles

Installing a vim plugin

$ git submodule add https://github.com/SirVer/ultisnips.git vim/pack/vendor/start/ultisnips

Updating vim plugins

$ git submodule update --init --remote

YouCompleteMe

Note there are significant installation steps depending on platform for YouCompleteMe https://github.com/ycm-core/YouCompleteMe#linux-64-bit