Add installation instructions for Homebrew and RubyGems to README

Author: andrew

Formula/git-pkgs.rb

@@ -0,0 +1,22 @@
+class GitPkgs < Formula
+  desc "Track package dependencies across git history"
+  homepage "https://github.com/andrew/git-pkgs"
+  url "https://github.com/andrew/git-pkgs/archive/refs/tags/v0.6.2.tar.gz"
+  sha256 "ccd7a8a5b9cb21c52cc488923ed1318387a9fefa4baff2057bd96b27591577aa"
+  license "AGPL-3.0"
+
+  depends_on "ruby@4"
+
+  def install
+    ENV["GEM_HOME"] = libexec
+    system "bundle", "install", "--without", "development"
+    system "gem", "build", "git-pkgs.gemspec"
+    system "gem", "install", "--ignore-dependencies", "git-pkgs-#{version}.gem"
+    bin.install libexec/"bin/git-pkgs"
+    bin.env_script_all_files(libexec/"bin", GEM_HOME: ENV.fetch("GEM_HOME", nil))
+  end
+
+  test do
+    system bin/"git-pkgs", "--version"
+  end
+end

README.md

@@ -12,6 +12,13 @@ It works across many ecosystems (Gemfile, package.json, Dockerfile, GitHub Actio
 
 ## Installation
 
+```bash
+brew tap andrew/git-pkgs
+brew install git-pkgs
+```
+
+Or with RubyGems:
+
 ```bash
 gem install git-pkgs
 ```