Added Slack logo. [ci skip]

Author: dblock

README.md

@@ -6,6 +6,8 @@ Slack-Ruby-Bot
 
 A generic Slack bot framework written in Ruby on top of [slack-ruby-client](https://github.com/dblock/slack-ruby-client). This library does all the heavy lifting, such as message parsing, so you can focus on implementing slack bot commands. It also attempts to introduce the bare minimum number of requirements or any sorts of limitations. It's a Slack bot boilerplate.
 
+![](slack.png)
+
 ## Usage
 
 ### A Minimal Bot