Clarify x86_64 support in README.md

Marks x86_64 support as host platform, with a footnote clarifying additional build time and requirements.

Partially addresses #20.

README.md

@@ -36,10 +36,15 @@ Linux distributions officially supported by the Swift project.
 | Platform       | Supported Version as Host | Supported Version as Target |
 | -:             | :-                        | :-                          |
 | macOS (arm64)  | ✅ macOS 13.0+            | ❌                         |
-| macOS (x86_64) | ⚠️ (WIP)                  | ❌                         |
+| macOS (x86_64) | ✅ macOS 13.0+[^1]        | ❌                         |
 | Ubuntu         | ⚠️ (WIP)                  | ✅ 20.04 / 22.04           |
 | RHEL           | ⚠️ (WIP)                  | ✅ UBI 9                   |
 
+
+[^1]: Since LLVM project doesn't provide pre-built binaries of `lld` for macOS on x86_64, it will be automatically built
+from sources by the generator, which will increase its run by at least 15 minutes on recent hardware. You will also
+need CMake and Ninja preinstalled (e.g. via `brew install cmake ninja`).
+
 ## How to use it
 
 Clone this repository into a directory of your choice and make it the current directory. Build and run it with this command: