Replace whats_new template in HAML with Markdown

lib/tasks/versions.rake

@@ -29,8 +29,8 @@ def render_whats_new(full_version)
   rubygems_version = Gem::Version.new(version).segments.map.with_index {|segment, i| i == 0 ? segment + 1 : segment }.join(".")
   date_slug = Date.today.strftime("%Y/%m/%d")
 
-  template = Pathname.new("../templates/whats_new.html.haml.erb").expand_path(__dir__)
+  template = Pathname.new("../templates/whats_new.html.md.erb").expand_path(__dir__)
   renderer = ERB.new(template.read)
-  whats_new = Pathname.new("../../source/#{full_version}/whats_new.html.haml").expand_path(__dir__)
+  whats_new = Pathname.new("../../source/#{full_version}/whats_new.html.md").expand_path(__dir__)
   File.write whats_new.to_s, renderer.result(binding)
 end

lib/templates/whats_new.html.md.erb

@@ -0,0 +1,16 @@
+# What's New in v<%= version %>
+
+The [Bundler <%= version %> announcement](/blog/<%= date_slug %>/bundler-v<%= version_slug %>.html)
+includes context and a more detailed explanation of the changes in this version.
+This is a summary of the biggest changes. As always, a detailed list of every change is provided in
+[the changelog](https://github.com/rubygems/rubygems/blob/<%= rubygems_version %>/bundler/CHANGELOG.md).
+
+## BIG NEW THING
+
+THE BIG NEW THING DOES COOL NEW STUFF.
+
+Bundler <%= version %> also includes:
+
+- SMALL NEW THING FIXES A PROBLEM
+
+<a href="https://github.com/rubygems/rubygems/blob/<%= rubygems_version %>/bundler/CHANGELOG.md" class="btn btn-primary">Full <%= version %> changelog</a>