Update README.md

Author: DmytroStepaniuk

README.md

@@ -30,6 +30,12 @@ Add variable to your stage
 set :mls_ruby_capistrano_slacker_webhook_url, ENV.fetch('CAPISTANO_SLACKER_WEBHOOK_URL')
 ```
 
+Also, if you want to publish release description to your Slack - just set following variable in your deploy configs
+
+```ruby
+set :mls_ruby_capistrano_slacker_post_release_description, true
+```
+
 ## Contributing
 
 1. Fork it