Add hubot-adapter topic

[bkeepers]

The list of hubot adapters in the hubot documentation is being replaced with a link to a GitHub topic search. Add the hubot-adapter topic to your repository on GitHub to include it in this list.

/cc hubotio/hubot#1320

[iangreenleaf]

Thanks for the note! This adapter is pretty much dead so I think I'll leave it off the list to avoid clutter.