refactor(externs): move Closure Externs back to Closure code repository

While Closure Compiler generally recommends to maintain the externs for
projects together with their source, this has not worked well for
AngularJS:
- Changes to externs must be tested; they can break clients. AngularJS
  has no testing infrastructure for this.
- Changes mostly come from users inside of Google and are much more
  easily submitted together with the code using them within Google's
  repository.

This change deletes the externs here and adds a README.closure.md to
document the change. They will be added back to Closure Compiler in a
separate submit.

Closes angular#5906

Author: mprobst

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/