feat: support install from homebrew (#360)

* feat: support install from homebrew

* docs: add readme.

HomebrewFormula/gorush.rb

@@ -0,0 +1,17 @@
+class Gorush < Formula
+  desc "A push notification server written in Go (Golang)."
+  homepage "https://github.com/appleboy/gorush"
+  head "https://github.com/appleboy/gorush.git"
+
+  depends_on "go" => :build
+
+  def install
+    ENV["GOPATH"] = buildpath
+    gorushpath = buildpath/"src/github.com/appleboy/gorush"
+    gorushpath.install buildpath.children
+    cd gorushpath do
+      system "go", "build", "-o", bin/"gorush"
+      prefix.install_metafiles
+    end
+  end
+end