refactor(externs): Move Closure Externs back to Closure code repository.

README.closure.md

@@ -0,0 +1,23 @@
+Using AngularJS with the Closure Compiler
+=========================================
+
+The Closure Compiler project contains externs definitions for AngularJS
+JavaScript in its `contrib/externs` directory.
+
+The definitions contain externs for use with the Closure compiler (aka
+JSCompiler). Passing these files to the --externs parameter of a compiler
+pass allows using type annotations for AngularJS objects. For example,
+Angular's $scope objects can be annotated as:
+```js
+/** @type {angular.Scope} */
+var scope = $scope;
+```
+
+This allows JSCompiler to type check accesses to scope, give warnings about
+missing methods or incorrect arguments, and also prevents renaming of property
+accesses with advanced compilation.
+
+The externs are incomplete and maintained on an as-needed basis, but strive to
+be correct. Externs for individual modules should be added in separate files.
+
+See https://developers.google.com/closure/compiler/