Add link to adapter for Symphony

[jonfreedman]

The symphony software foundation are working on a hubot adapter for the Symphony chat system @ https://github.com/symphonyoss/hubot-symphony

It would be great to get this listed on the docs

[jonfreedman]

FYI the adapter is fully functional - we're using it live

[bkeepers]

Add hubot-adapter topic

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 #1320