docs(vendor): add readme

Author: Narretz

vendor/README.md

@@ -0,0 +1,27 @@
+# Vendor Libraries
+
+Libraries that are not available via yarn / the npm registry, are checked into git in the `vendor`
+folder.
+
+Currently this affects the following libraries:
+
+## closure-compiler
+
+Version: `20140814`
+
+The closure-compiler is available on npm, but not the version we are using.
+
+## ng-closure-runner
+
+Version: `0.2.4`
+
+This project has never been published to npm.
+
+# Updating the libraries
+
+If a different version becomes available, it must be manually downloaded and replaced in the
+repository.
+
+Should yarn support requiring zip archives in the future (see
+[yarn github issue: Zip support](https://github.com/yarnpkg/yarn/issues/1483)),
+we can remove the libraries from the repository and add them to the package.json.