README: brew trust step + sportsball example

@@ -4,8 +4,9 @@ Self-hosted [Homebrew](https://brew.sh) tap — install these tools without GitHub.
 
 ```sh
 brew tap kortum/tap https://git.kortum.world/git/homebrew-tap.git
-brew install <formula>
+brew trust kortum/tap        # recent Homebrew gates third-party taps
+brew install <formula>       # e.g. sportsball
 ```
 
 Formulas live in `Formula/` and build from source (`depends_on "go"`). Released
-via custard's `scripts/brew-release.sh`.
+via custard's `scripts/brew-release.sh`. Upgrade later with `brew update && brew upgrade <formula>`.