docs: Provide example setup for use-package on Emacs 30+

[tedroden]

Newer versions of use-package provide a :vc keyword. Users can now install copilot.el on standard emacs without quelpa or straight.

That version of use-package is already in Emacs 30.

Updated README.md to show an example.

(Thank you so much for this package!)

[zerolfx]

GitHub doesn't support #######. Could you use bold or plain text instead?

[tedroden]

Good catch. Fixed.

[jcs090218]

Thanks for the contribution! :D