Add missing macOS dependencies to Brewfile, update README.md

Brewfile

@@ -1 +1,4 @@
 brew 'xz'
+brew 'zstd'
+brew 'cmake'
+brew 'ninja'

README.md

@@ -24,19 +24,22 @@ After that, verify that the `experimental-sdk` command is available:
 swift experimental-sdk list
 ```
 
-The output will either state that no Swift SDKs are available, or produce a list of those you previously had 
-installed, in case you've used the `swift experimental-sdk install` command before.
+The output will either state that no Swift SDKs are available, or produce a list of those you previously had installed, in case you've used the `swift experimental-sdk install` command before.
 
 ### macOS Requirements
 
-The generator depends on the `xz` utility for more efficient downloading of package lists for Ubuntu. This is optional, but can be installed via the included `Brewfile`:
+The generator depends on the following dependencies to be installed on macOS:
+
+- `xz`: used for more efficient downloading of package lists for Ubuntu. If `xz` is not found, the generator will fallback on `gzip`.
+- `cmake` and `ninja`: required for building LLVM native for versions of Swift before 6.0. _*NOTE: if you're building the Swift toolchain outside of Swift SDK Generator, you'll have to uninstall Homebrew CMake or remove it from your `PATH` before invoking toolchain's `build-script` due to [swiftlang/swift#83060](https://github.com/swiftlang/swift/pull/83060).*_
+- `zstd`: required to decompress certain downloaded artifacts that use [Zstandard](https://github.com/facebook/zstd) compression.
+
+These dependencies can be installed from the `Brewfile`:
 
 ```bash
 brew bundle install
 ```
 
-If `xz` is not found, the generator will fallback on `gzip`.
-
 ## Supported platforms and minimum versions
 
 macOS as a host platform and Linux as both host and target platforms are supported by the generator.