Add Hexdocs link to generated new project README (#4906)

Author: lpil

lib/mix/lib/mix/tasks/new.ex

@@ -222,6 +222,9 @@ defmodule Mix.Tasks.New do
         [applications: [:<%= @app %>]]
       end
       ```
+
+  If [published on Hexdocs](https://hex.pm/docs/tasks#hex_docs), the docs can
+  be found at [https://hexdocs.pm/<%= @app %>](https://hexdocs.pm/<%= @app %>)
   <% end %>
   """