Updated README clarifying usage of gem. [ci skip]
Updated README clarifying usage of gem. [ci skip]

dblock/slack-ruby-bot-server,dblock/slack-bot-server,dblock/slack-bot-server,dblock/slack-ruby-bot-server,dblock/slack-ruby-bot-server,dblock/slack-bot-server,slack-ruby/slack-ruby-bot-server,slack-ruby/slack-ruby-bot-server,slack-ruby/slack-ruby-bot-server,dblock/slack-bot-server