Add instruction for metal to Getting Started

[kevinbader]

gpui requires "metal", but it's not installed on a new system. The instruction should resolve a stumbling block for other newcomers like me, and comes from https://github.com/gfx-rs/gfx/issues/2309#issuecomment-413695219

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[maxdeviant]

Thanks for the PR. I'm going to close in favor of https://github.com/zed-industries/zed/pull/17316.