Merge pull request #12170 from siong1987/task

[ci skip] Documentation on how to create custom rake tasks.

guides/source/command_line.md

@@ -493,7 +493,9 @@ The `tmp:` namespaced tasks will help you clear and create the `Rails.root/tmp`
 
 ### Custom Rake Tasks
 
-Custom rake tasks have a `.rake` extension and are placed in `Rails.root/lib/tasks`.
+Custom rake tasks have a `.rake` extension and are placed in
+`Rails.root/lib/tasks`. You can create these custom rake tasks with the `rails
+generate task` command.
 
 ```ruby
 desc "I am short, but comprehensive description for my cool task"