Add 3rd helm action to show extra doc.

* helm-j-cheatsheet.el (jc-make-insert-primary): New custom variable.
  (jc-others): New dictionary section.
  (jc-action-show-2nd-doc): New function.
  (jc-action-transformer): New function.
  (jc-candidates): add action-transformer.
  (helm-source-j-cheatsheet): add "Others" secion.
Add README.