Added a note in RELEASING on removing the section about the stable release. [ci skip]
Added a note in RELEASING on removing the section about the stable release. [ci skip]

slack-ruby/slack-ruby-client,rkadyb/slack-ruby-client,slack-ruby/slack-ruby-client,mikz/slack-ruby-client,mikz/slack-ruby-client,dblock/slack-ruby-client,jlyonsmith/slack-ruby-client,rkadyb/slack-ruby-client,jlyonsmith/slack-ruby-client,dblock/slack-ruby-client