Update README.md to indicate deprecation

The homebrew-macvim repository should be officially considered deprecated, as Homebrew Core has its own version of a formula for MacVim. This could be revisited if there's a good reason to maintain our own formula but now it is additional ongoing maintenance work and misleading to new users.

Author: ychin

README.md

@@ -1,5 +1,9 @@
 # Homebrew Tap for MacVim [![Build Status](https://travis-ci.org/macvim-dev/homebrew-macvim.svg?branch=master)](https://travis-ci.org/macvim-dev/homebrew-macvim)
 
+## Note: Deprecation
+
+This repo is currently deprecated. Homebrew Core maintains its own formula for MacVim (https://github.com/Homebrew/homebrew-core/blob/master/Formula/macvim.rb) and it is actively maintained and up to date. Consider using it in favor of this one.
+
 ## Usage
 
 ### One-time setup