Add instruction for metal to Getting Started

gpui requires "metal", but it's not installed on a new system. The fix comes from gfx-rs/gfx#2309 (comment)

crates/gpui/README.md

@@ -5,7 +5,9 @@ for Rust, designed to support a wide variety of applications.
 
 ## Getting Started
 
-GPUI is still in active development as we work on the Zed code editor and isn't yet on crates.io. You'll also need to use the latest version of stable rust and be on macOS. Add the following to your Cargo.toml:
+GPUI is still in active development as we work on the Zed code editor and isn't yet on crates.io. You'll also need to use the latest version of stable rust and be on macOS. If `xcrun metal` is _unable to find utility "metal"_, install `xcode` and run `sudo xcode-select --switch /Applications/Xcode.app/Contents/Developer`.
+
+Add the following to your Cargo.toml:
 
 ```toml
 gpui = { git = "https://github.com/zed-industries/zed" }