feat: add custom post type for letters to the editor

Updated README to mention the new custom post type for letters to the editor. Adjusted function naming in `functions.php` to reflect the new feature. This enhancement allows users to manage and display editorial letters more effectively.

README.md

@@ -14,3 +14,5 @@ It includes:
       - Prefix configurable in Customizer (please change this to your own prefix)
     - DOI link on the article page
     - DOI resolver (https://example.com/nn.nnnn/xyz* forwards to the respective article)
+
+- Custom post type for letters to the editor

functions.php

@@ -457,7 +457,7 @@ function create_letter_post_type() {
 }
 add_action('init', 'create_letter_post_type');
 
-function add_article_meta_boxes() {
+function add_letter_meta_boxes() {
     add_meta_box(
         'article_authors',
         __('Authors', 'duck-behavior-journal'),