release-notes: add message directing users to the blog.

Library/Homebrew/dev-cmd/release-notes.rb

@@ -58,6 +58,7 @@ module Homebrew
     end
 
     $stderr.puts "Release notes between #{previous_tag} and #{end_ref}:"
+    puts "Release notes for major and minor releases can be found in the [Homebrew blog](https://brew.sh/blog/)."
     puts output
   end
 end