Add README with install and usage instructions

README.md

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+# Homebrew Tap for vardrun
+
+## Install
+
+```bash
+brew tap ijohanne/vardrun https://git.unixpimps.net/ijohanne/vardrun-homebrew.git
+brew install vardrun
+```
+
+## Upgrade
+
+```bash
+brew update
+brew upgrade vardrun
+```
+
+## Uninstall
+
+```bash
+brew uninstall vardrun
+brew untap ijohanne/vardrun
+```
+
+## Supported platforms
+
+| OS    | Architecture |
+|-------|-------------|
+| macOS | Apple Silicon (arm64) |
+| macOS | Intel (amd64) |
+| Linux | arm64 |
+| Linux | amd64 |
+
+## Publishing new releases
+
+From the vardrun source repo, run:
+
+```bash
+GITEA_HOMEBREW_API_TOKEN=<token> nix run .#homebrew-release
+```
+
+This builds a static binary for the current platform, uploads it to the Gitea package registry, and updates the formula. Run once per target platform to populate all sha256 hashes.