Add user_defined_tags.

Author: heavenshell

README.md

@@ -37,6 +37,7 @@ Option                                  | Default | Description
 **g:jsdoc_type_hook**                   | {}      | Allow to insert default description depending on the type.
 **g:jsdoc_enable_es6**                  | 0       | Enable to use ECMAScript6's Shorthand function, Arrow function.
 **g:jsdoc_tags**                        | see :h  | Allow use of alternate tags (the ones that support synonyms) per JSDoc documentation. Can be changed on a per tag basis, for example: `let g:jsdoc_tags = {} | let g:jsdoc_tags['param'] = 'arg'`
+**g:jsdoc_user_defined_tags**           | {}     | Allow use of `user_defined_tags`.
 
 ## Keymap
 Since version `0.3`, `g:jsdoc_default_mapping` was removed.