Add the instructions for updating the package to README.md

[dherrera1911]

I want to update copilot.vim to the latest version, but there are no instructions here. I will likely figure it out with a bit of research, but it would be very helpful to have this info in the README.

[yermulnik]

I guess it should be as simple as doing git pull inside the dir where you cloned this repo initially. Something similar to this granted you did the git clone --depth=1 https://github.com/github/copilot.vim.git ~/.vim/pack/github/start/copilot.vim initially:

git -C ~/.vim/pack/github/start/copilot.vim pull

[dherrera1911]

Thanks. I think it is as simple as just pulling. I went to ~/.vim/pack/github/start/copilot.vim and did git pull and now in vim :Copilot version shows the latest version (1.59.0). Still, I think it would be useful to have here, so a user like me doesn't go around wondering if they need to run :Copilot setup again or what.

[yermulnik]

Agree, that makes sense. But I'm not part of this repo though. Let's see if @tpope could pick this up 👍🏻