Update README with Homebrew instructions.

Leela Zero is now in Homebrew. Update macOS instructions
accordingly.

Pull request leela-zero#2348.

README.md

@@ -72,7 +72,13 @@ a (weaker) network trained from human games [here](https://sjeng.org/zero/best_v
 If you are on Windows, download an official release from [here](https://github.com/leela-zero/leela-zero/releases) and head to the [Usage](#usage-for-playing-or-analyzing-games)
 section of this README.
 
-If you are on Unix or macOS, you have to compile the program yourself. Follow
+If you are on macOS, Leela Zero is available through [Homebrew](https://homebrew.sh), the de facto standard
+package manager. You can install it with:
+```
+brew install leela-zero
+```
+
+If you are on Unix, you have to compile the program yourself. Follow
 the compilation instructions below and then read the [Usage](#usage-for-playing-or-analyzing-games) section.
 
 # Compiling AutoGTP and/or Leela Zero