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

[chrisdotcode]

Thanks! Whoops, looks like I don't have "add tag" permissions as of yet on the repo.

paging @nandub for that access