Adding link to copilot-chat.el project in README

[tninja]

Currently copilot.el don't have chat capability. But github plugin for IntelliJ and VSCode have.

I happen to find the copilot-chat.el project on github. It might be less known than copilot.el. I tried that, it works well. I think it worth to have a link in the README, it is complimentary to copilot.el, and could be useful to people.