feat: support install from homebrew (appleboy#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

README.md

@@ -194,6 +194,12 @@ On Windows
 $ wget https://github.com/appleboy/gorush/releases/download/1.10.0/gorush-1.10.0-windows-amd64.exe -O gorush.exe
 ```
 
+On macOS, use Homebrew.
+
+```
+$ brew install --HEAD https://github.com/appleboy/gorush/raw/master/HomebrewFormula/gorush.rb
+```
+
 ### Command Usage
 
 ```