Provide a link to third party web components

The WebComponents.org catalog seems to enforce open source, non commercial libraries to be added. I'm perfectly fine with that.

But as a central place to find web components, I think it would help foster more interest in web components to have an additional page where third-party web components that might not fit the open source policies of the registry might be found. Things like [Onsen UI](https://onsen.io/) or the [polymer wrapper around HTMLTreeGrid.](https://github.com/flexicious/webcomponents-datagrid-polymer-starter-kit).