Create package for HTML imports

pkg/html_import/README.md

+# HTML Imports polyfill
+
+[HTML Imports][1] are a way to include and reuse HTML documents in other HTML
+documents. As `<script>` tags let authors include external Javascript in their
+pages, imports let authors load full HTML resources.  In particular, imports let
+authors include [Custom Element](https://github.com/Polymer/CustomElements)
+definitions from external URLs.
+
+
+## Getting Started
+
+Include the `html_import.debug.js` or `html_import.min.js` (minified) file in
+your project.
+
+    <script src="packages/html_import/html_import.debug.js"></script>
+
+`html_import.debug.js` is the debug loader and uses `document.write` to load
+additional modules.
+
+Use the minified version (`html_import.min.js`) if you need to load the file
+dynamically.
+
+## Basic usage
+
+For HTML imports use the `import` relation on a standard `<link>` tag, for
+example:
+
+    <link rel="import" href="import-file.html">
+
+## Polyfill details
+
+You can read more about how the polyfill is implemented in JavaScript here:
+<https://github.com/Polymer/HTMLImports/tree/master#polyfill-details>
+
+## Getting the source code
+
+This package is built from:
+<https://github.com/Polymer/HTMLImports/tree/master>
+
+You'll need [node.js](http://nodejs.org) to rebuild the JS file. Use
+`npm install` to get dependencies and `grunt` to build.
+
+[1]: https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/imports/index.html