Explain how macvim differs from vanilla vim

I couldn't find anywhere (README, website, FAQ) what macvim is about and how it relates to the official Vim project.

Is your feature request related something that is currently hard to do? Please describe.
It is hard to understand what macvim is. The website only mentions "Vim - the text editor - for macOS", which is confusing given that 1) Vim already ships with macOS and 2) there already is a Homebrew recipe for Vim.

Describe the solution you'd like
I would like a concise description of the project to be added, at least in the README.

Describe alternatives you've considered
N/A

Additional context
N/A

まず README、ウェブサイト、FAQ を確認し、現在 MacVim がどのように説明され、macOS 向けの Vim がどのように紹介されているかを把握します。MacVim とは何か、vanilla Vim とどう異なるのか、公式の Vim プロジェクトとどのような関係にあるのかを簡潔に説明する内容を追加します。README からプロジェクトの目的が明確に分かれば完了です。