Add comparison with 'slack-api' library

[chshersh]

There's another library for Slack API bindings:

• https://hackage.haskell.org/package/slack-api
• https://github.com/owainlewis/slack

It would be good if you add comparison with that library in your README. Otherwise it's not easy to decide for developers which library to use.

[hlian]

i'd gladly accept a pr!