docs: explain directory argument to :Bundle
docs: explain directory argument to :Bundle

Add some documentation to explain the directory argument (optional second
argument) to the Bundle command.  Maybe fixes #295

kotamat/vundle,omalsa04/dotfiles,ppslinux/Vundle.vim,ppslinux/Vundle.vim,kotamat/vundle,ppslinux/Vundle.vim,ppslinux/Vundle.vim,TLmaK0/my_vim_config,kotamat/vundle,stephaneliu/dotfiles,kotamat/vundle,ppslinux/Vundle.vim,kotamat/vundle,omalsa04/dotfiles,sensorario/Vundle.vim,ppslinux/Vundle.vim,stephaneliu/dotfiles