Support Vundle or add installation instruction

[belkka]

I tried to install this plugin using Vundle by adding a Plugin 'kana/vim-textobj-function' line to init.vim. But plugin doesn't work and there is an error message on vim startup:

Error detected while processing /home/ulianich/.config/nvim/bundle/vim-textobj-function/plugin/textobj/function.vim:
line 32:
E117: Unknown function: textobj#user#plugin

It would be great either to fix this, or add an installation instruction in README

[ayoubelmhamdi]

also not be installed with packer
maybe we need to install kana/vim-textobj-user