docs: mention brew install in the README

wrkflw has been in homebrew-core for a while now, but the README
kept pointing everyone at cargo as if that were the only way in.
Not everyone has a Rust toolchain sitting around, and telling
someone to go install one just to try the tool is mildly hostile.

Add a brew install line right under the cargo one, linked to the
formula page. That's it.

README.md

@@ -43,6 +43,12 @@ A command-line tool for validating and executing GitHub Actions workflows locall
 cargo install wrkflw
 ```
 
+Or with Homebrew ([formula](https://formulae.brew.sh/formula/wrkflw)):
+
+```bash
+brew install wrkflw
+```
+
 Or build from source:
 
 ```bash