Add proper installation instruction for Doom Emacs in README.md

[Jintekki]

The README advises installing via M-x evil-tutor-start, but Doom Emacs (a very popular configuration framework for Evil Mode) does not use the same package manager as vanilla Emacs.

WARNING: Do not install packages directly (with M-x package-install or M-x straight-use-package). Without an accompanying package! declaration somewhere these packages will be forgotten when you restart Emacs and uninstalled the next time you run doom sync or doom purge.

Instead, the following line should be added in your ~/.doom.d/packages.el file: (package! evil-tutor), then a doom sync should be ran.

[eallenOP]

Thanks, this is the only help in this whole repo for how to actually install this tutor for Doom users.