Add notice of repo migration to README

Author: maxbrunsfeld

README.md

@@ -1,6 +1,10 @@
 # Git Diff package
 [![OS X Build Status](https://travis-ci.org/atom/git-diff.svg?branch=master)](https://travis-ci.org/atom/git-diff) [![Windows Build Status](https://ci.appveyor.com/api/projects/status/9auj52cs0vso66nv/branch/master?svg=true)](https://ci.appveyor.com/project/Atom/git-diff/branch/master) [![Dependency Status](https://david-dm.org/atom/git-diff.svg)](https://david-dm.org/atom/git-diff)
 
+### This package is now a part of the [core Atom repository](https://github.com/atom/atom/tree/master/packages/git-diff), please direct all issues and pull requests there in the future!
+
+---
+
 Marks lines in the editor gutter that have been added, edited, or deleted since the last commit.
 
   * <kbd>alt-g up</kbd> to move the cursor to the previous diff in the editor