README.md: Mention Homebrew/homebrew-core#210511

Author: simnalamburt

README.md

@@ -7,6 +7,9 @@ brew install simnalamburt/x/ra-multiplex
 brew services start simnalamburt/x/ra-multiplex
 ```
 
+> **NOTE**: A PR is currently in progress to migrate ra-multiplex to
+> homebrew-core. https://github.com/Homebrew/homebrew-core/pull/210511
+
 ### [input-source](https://github.com/simnalamburt/macos-input-source)
 Tiny input source manager for macOS.
 ```bash