docs: performance hint for doom emacs users

[pmbittner]

This commit adds a hint to the README for doom emacs users.

Customizing the flavor of cattpuccin in doom emacs as suggested in the README:

(load-theme 'catppuccin t t)
(setq catppuccin-flavor 'frappe) ;; or 'latte, 'macchiato, or 'mocha
(catppuccin-reload)

increases the startup time of doom emacs significantly (likely because catppuccin is loaded twice).
I noticed that setting the flavor before loading the theme:

(setq catppuccin-flavor 'frappe) ;; or 'latte, 'macchiato, or 'mocha
(load-theme 'catppuccin t t)

works and so the call to (catppuccin-reload) can be omitted, yielding fast startup times again.

I added a respective hint for doom users to the README.