Add link with info about how to register a new htmlwidget

[hafen]

No description provided.

[jpmarindiaz]

+1

[hafen]

I'll work on this. For now, here's the quick version:

1. Fork this repository
2. Add a new entry in _config.yml for your widget (see the others in the config for examples) and a thumbnail in the images directory
3. Submit a pull request

An alternative is to send me the info that needs to go in _config.yml and the thumbnail (could do this through a github issue) and I can add it in.