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

[geekgonecrazy]

@bkeepers thanks for the heads-up! I've added the topic to the repo and now shows up in the topic search 👍