Add badges for Homebrew / MacPorts packages, and download link

Similar to Vim, add badges for packages to have easy link to their
respective repositories in their package managers. Since we only have 2
main package managers in Mac, individually link to them and their
respective versions.

Also add a direct link to download binary releases for convenience and
clarity.

Author: ychin

README.md

@@ -3,7 +3,11 @@ Vim - the text editor - for Mac OS X
 
 - MacVim homepage https://macvim-dev.github.io/macvim
 
-- Vim README https://github.com/macvim-dev/macvim/blob/master/README_vim.md
+- Download the latest version from [Releases](https://github.com/macvim-dev/macvim/releases/latest).
+
+- Vim README: [README_vim.md](README_vim.md)
 
 - Travis CI <a href="https://travis-ci.org/macvim-dev/macvim"><img src="https://travis-ci.org/macvim-dev/macvim.svg?branch=master" alt="Build Status"></a><a href="https://travis-ci.org/macvim-dev/homebrew-macvim"><img src="https://travis-ci.org/macvim-dev/homebrew-macvim.svg?branch=master" alt="Build Status"></a>
 
+- Packaged in [![Homebrew package](https://repology.org/badge/version-for-repo/homebrew/macvim.svg)](https://repology.org/metapackage/macvim/versions) [![MacPorts package](https://repology.org/badge/version-for-repo/macports/macvim.svg)](https://repology.org/metapackage/macvim/versions)
+